To clarify, this applies to both GN and GYP builds. But it might be easier to do this transition after GYP is deprecated (half work :))

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2476ed2fc01e9d8a9603e8b3bb7e7bd311e26afa

commit 2476ed2fc01e9d8a9603e8b3bb7e7bd311e26afa
Author: xhwang <xhwang@chromium.org>
Date: Thu Mar 24 17:39:25 2016

media: Fix typo in USE_PROPRIETARY_CODECS check.

This is a good example why BUILDFLAG would be better. See  issue 596252 
for the bug to track using BUILDFLAG in media.

BUG= 596252 
TEST=Initdata parsing works now.

Review URL: https://codereview.chromium.org/1828153002

Cr-Commit-Position: refs/heads/master@{#383089}

[modify] https://crrev.com/2476ed2fc01e9d8a9603e8b3bb7e7bd311e26afa/components/cdm/browser/widevine_drm_delegate_android.cc

As we switch mechanisms, it might be a good time to separate support for proprietary codecs from some of the containers that contain them (both currently controlled by USE_PROPRIETARY_CODECS). For example,  issue 327115 .

 Issue 605182  has been merged into this issue.

Most buildflags are simple mapping from gn args. However, mojo media does use something fancy, e.g. define macros based on the elements of a list:

https://cs.chromium.org/chromium/src/media/mojo/services/BUILD.gn?rcl=0&l=10

I don't see existing examples how to do this. 

brettw: Before I dive in and try do you have any idea on how to do this? Or are there any examples to follow? Thanks!

GN doesn't do easy inclusion-based tests. Can you describe the design?

It's described here:

https://cs.chromium.org/chromium/src/media/media_options.gni?rcl=0&l=98

For example, I can specify this gn arg:

mojo_media_services = [
  "cdm",
  "audio_decoder",
]

Today it'll define ENABLE_MOJO_CDM and ENABLE_MOJO_AUDIO_DECODER [1]. Note that I use foreach to get each element of the list, and then check each element to define the macro. I wonder whether I can do something similar with buildflags.

[1] https://cs.chromium.org/chromium/src/media/mojo/services/BUILD.gn?rcl=0&l=23

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/59123b612a37738072b80870dfd18a7e8eaae241

commit 59123b612a37738072b80870dfd18a7e8eaae241
Author: xhwang <xhwang@chromium.org>
Date: Fri Jan 06 01:19:42 2017

media: Fix ENABLE_MOJO_MEDIA_IN_BROWSER_PROCESS check

Replace

 if (ENABLE_MOJO_MEDIA_IN_BROWSER_PROCESS)

with

 if defined(ENABLE_MOJO_MEDIA_IN_BROWSER_PROCESS)

The former was working because some compilers predefine FOO to be 1 when
-DFOO is specified. For example:
https://gcc.gnu.org/onlinedocs/gcc/Preprocessor-Options.html

But I am not sure whether this is true for all compilers. Hence we
probably should not depend on it.

Long term, we should switch to use BUILDFLAG to avoid this.

TBR=mkwst@chromium.org
BUG= 596252 

Review-Url: https://codereview.chromium.org/2616703005
Cr-Commit-Position: refs/heads/master@{#441782}

[modify] https://crrev.com/59123b612a37738072b80870dfd18a7e8eaae241/chrome/browser/chrome_content_browser_client.cc
[modify] https://crrev.com/59123b612a37738072b80870dfd18a7e8eaae241/content/shell/browser/shell_content_browser_client.cc

The build flags just need to end up with a list in the buildflag_header target. So any way you can generate that list will work, including something similar to the way you used to do it.

Cool, thanks! I'll give it a try.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9d5f354616dea455ea08345c96b67081275a865d

commit 9d5f354616dea455ea08345c96b67081275a865d
Author: xhwang <xhwang@chromium.org>
Date: Sat May 13 03:57:41 2017

media: Add mojo_media_config to media/gpu configs

Otherwise, macros like ENABLE_MOJO_MEDIA_IN_GPU_PROCESS are not defined.
We should switch to use BUILDFLAG to prevent this from happening, that
is tracked by  issue 596252 .

BUG= 596252 ,721935
TEST=Manually tested encrypted playback.
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2876123004
Cr-Commit-Position: refs/heads/master@{#471553}

[modify] https://crrev.com/9d5f354616dea455ea08345c96b67081275a865d/media/gpu/BUILD.gn

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/db24d60f03a20d0efbe35a390def732c6105b403

commit db24d60f03a20d0efbe35a390def732c6105b403
Author: xhwang <xhwang@chromium.org>
Date: Sat May 20 06:32:24 2017

media: Convert mojo media defines to buildflags

BUG= 596252 
TEST=No functionality change
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2882813002
Cr-Commit-Position: refs/heads/master@{#473421}

[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/chrome/browser/BUILD.gn
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/chrome/browser/chrome_content_browser_client.cc
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/chromecast/browser/cast_content_browser_client.cc
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/content/browser/BUILD.gn
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/content/browser/browser_main_loop.cc
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/content/browser/media/encrypted_media_browsertest.cc
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/content/browser/media/media_interface_proxy.cc
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/content/browser/renderer_host/render_process_host_browsertest.cc
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/content/browser/service_manager/service_manager_context.cc
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/content/browser/webrtc/webrtc_capture_from_element_browsertest.cc
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/content/gpu/BUILD.gn
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/content/gpu/gpu_service_factory.cc
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/content/gpu/gpu_service_factory.h
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/content/renderer/BUILD.gn
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/content/renderer/render_frame_impl.cc
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/content/renderer/render_frame_impl.h
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/content/shell/BUILD.gn
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/content/shell/browser/shell_content_browser_client.cc
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/content/shell/renderer/shell_content_renderer_client.cc
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/content/shell/renderer/shell_content_renderer_client.h
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/content/utility/BUILD.gn
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/content/utility/utility_service_factory.cc
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/media/gpu/BUILD.gn
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/media/gpu/android_video_decode_accelerator.cc
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/media/mojo/BUILD.gn
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/media/mojo/clients/BUILD.gn
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/media/mojo/clients/mojo_cdm_factory.cc
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/media/mojo/clients/mojo_decoder_factory.cc
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/media/mojo/services/BUILD.gn
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/media/mojo/services/interface_factory_impl.cc
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/media/mojo/services/interface_factory_impl.h
[modify] https://crrev.com/db24d60f03a20d0efbe35a390def732c6105b403/media/mojo/services/media_service_unittest.cc

We still have a few defines to be converted. I'll keep this bug open until those are fixed as well.

https://cs.chromium.org/search/?q=define+file:BUILD.gn+file:%5Esrc/media/+package:%5Echromium$&type=cs

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bcf523b9595b64c082fceade6f9900b9a8723cda

commit bcf523b9595b64c082fceade6f9900b9a8723cda
Author: Dale Curtis <dalecurtis@chromium.org>
Date: Wed Jan 17 02:59:01 2018

Switch various media #defines to BUILDFLAGS.

Converts the following to buildflags:
- MEDIA_DISABLE_FFMPEG becomes ENABLE_FFMPEG, but keeps media_use_ffmpeg
as the GN variable due to external users.
- MEDIA_DISABLE_LIBVPX becomes ENABLE_LIBVPX, but keeps media_use_libvpx
as the GN variable due to external users.
- DISABLE_FFMPEG_VIDEO_DECODERS becomes ENABLE_FFMPEG_VIDEO_DECODERS and
the GN variable changes to enable_ffmpeg_video_decoders due to no
external users AFAIK.

Some incorrect usages of just MEDIA_DISABLE_FFMPEG to gate H264 support
and some less-efficient usages of !MEDIA_DISABLE_FFMPEG + !ANDROID are
replaced with the correct ENABLE_FFMPEG_VIDEO_DECODERS instead.

BUG= 596252 
TEST=compiles normally

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Iab4548feabab60d7fbed4144457f0be9f2e98f47
Reviewed-on: https://chromium-review.googlesource.com/865520
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#529559}
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/chrome/browser/about_flags.cc
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/chrome/browser/media/encrypted_media_supported_types_browsertest.cc
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/chrome/services/media_gallery_util/BUILD.gn
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/chrome/services/media_gallery_util/media_metadata_parser.cc
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/chrome/services/media_gallery_util/media_parser.cc
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/content/browser/media/media_canplaytype_browsertest.cc
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/content/public/common/BUILD.gn
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/content/public/common/feature_h264_with_openh264_ffmpeg.cc
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/content/public/common/feature_h264_with_openh264_ffmpeg.h
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/content/renderer/media/audio_decoder.cc
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/content/renderer/media/gpu/rtc_video_encoder_factory.cc
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/content/renderer/media/webrtc/peer_connection_dependency_factory.cc
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/content/renderer/pepper/video_decoder_shim.cc
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/media/BUILD.gn
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/media/base/BUILD.gn
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/media/base/decode_capabilities.cc
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/media/base/media.cc
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/media/base/mime_util_internal.cc
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/media/blink/webmediaplayer_impl.cc
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/media/ffmpeg/ffmpeg_common.cc
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/media/filters/BUILD.gn
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/media/filters/vpx_video_decoder.cc
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/media/media_options.gni
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/media/renderers/BUILD.gn
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/media/renderers/default_renderer_factory.cc
[modify] https://crrev.com/bcf523b9595b64c082fceade6f9900b9a8723cda/media/test/pipeline_integration_test_base.cc

All the defines which wiggled their way outside of media/ are fixed, there are a couple media internal ones like USE_CRAS, USE_NEON, etc:

https://cs.chromium.org/chromium/src/media/BUILD.gn?l=47

This issue has been Available for over a year. If it's no longer important or seems unlikely to be fixed, please consider closing it out. If it is important, please re-triage the issue.

Sorry for the inconvenience if the bug really should have been left as Available.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Marking as fixed, the remaining ones are internal only defines for audio code. They can be converted, but aren't entirely necessary.