WIP patch: 

Mark, do you still expect to land this for M56?  If not please update milestone.

Mark is this still in your queue?  If not please punt back to Available.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6a8d2a721d7d281dba0bb87fbd1d0e9358d6ce7d

commit 6a8d2a721d7d281dba0bb87fbd1d0e9358d6ce7d
Author: mark a. foltz <mfoltz@chromium.org>
Date: Thu Sep 21 17:27:42 2017

[Desktop Capture] Enable system audio capture without screen capture.

This addresses a long standing issue where users are unable to cast system
audio to audio-only Cast devices by adding support for system audio capture
without screen capture.

Bug: 662499
Change-Id: I0c612861e36644298f161de47810c6d37ffbf501
Reviewed-on: https://chromium-review.googlesource.com/656597
Commit-Queue: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#503491}
[modify] https://crrev.com/6a8d2a721d7d281dba0bb87fbd1d0e9358d6ce7d/chrome/app/generated_resources.grd
[modify] https://crrev.com/6a8d2a721d7d281dba0bb87fbd1d0e9358d6ce7d/chrome/browser/media/webrtc/desktop_capture_access_handler.cc
[modify] https://crrev.com/6a8d2a721d7d281dba0bb87fbd1d0e9358d6ce7d/content/browser/renderer_host/media/media_stream_manager.cc
[modify] https://crrev.com/6a8d2a721d7d281dba0bb87fbd1d0e9358d6ce7d/content/public/browser/desktop_media_id.cc
[modify] https://crrev.com/6a8d2a721d7d281dba0bb87fbd1d0e9358d6ce7d/content/public/browser/desktop_media_id.h
[modify] https://crrev.com/6a8d2a721d7d281dba0bb87fbd1d0e9358d6ce7d/content/renderer/media/media_stream_dispatcher_unittest.cc

When using `chrome.desktopCapture.chooseDesktopMedia(['audio'], function(desktopMediaRequestId, options) { })` currently, the [`kEmptySourcesListError` -> `At least one source type must be specified.` error](https://chromium.googlesource.com/chromium/src/+/6a8d2a721d7d281dba0bb87fbd1d0e9358d6ce7d/chrome/browser/extensions/api/desktop_capture/desktop_capture_base.cc#40) is thrown because `api::desktop_capture::DESKTOP_CAPTURE_SOURCE_TYPE_AUDIO` doesn't add anything to the `source_lists` and you need to accompany with with some video capture like `['screen', 'audio']`.

@Mark In https://chromium.googlesource.com/chromium/src.git/+/6a8d2a721d7d281dba0bb87fbd1d0e9358d6ce7d%5E%21/, I don't see any change to `DesktopCaptureChooseDesktopMediaFunctionBase::Execute`(`desktop_capture_base.cc`) and would still result in the `kEmptySourcesListError` error if you only used `['audio']` while your commit makes it seem like it may be possible now.

Perhaps those changes mean you only need to provide audio constraints now when using `getUserMedia, `navigator.webkitGetUserMedia({ audio: { mandatory: { chromeMediaSource: 'system' } }, function(mediaStream) { })`

Sorry if you addressing another part of the code, I'm not very familiar with how everything connects.

We've verified internally that this works in M63.

Sorry didn't see C#15.  The patch didn't add support for audio-only capture to chrome.desktopCapture.chooseDesktopMedia as it wasn't required to fix the blocking issue for casting from Chrome.  If you'd like to do that then further changes are involved and probably new UX.  I would suggest filing a separate feature request under Extensions>API and Blink>GetUserMedia>Desktop.

Thanks @Mark! I have filed a new issue here, https://bugs.chromium.org/p/chromium/issues/detail?id=769388

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/979d466da3ae3d78c6a855fb6e1915be9c0e12fc

commit 979d466da3ae3d78c6a855fb6e1915be9c0e12fc
Author: mark a. foltz <mfoltz@chromium.org>
Date: Fri Sep 29 17:05:40 2017

Revert "[Desktop Capture] Enable system audio capture without screen capture."

This reverts commit 6a8d2a721d7d281dba0bb87fbd1d0e9358d6ce7d.

Reason for revert: Bug 769979

Original change's description:
> [Desktop Capture] Enable system audio capture without screen capture.
> 
> This addresses a long standing issue where users are unable to cast system
> audio to audio-only Cast devices by adding support for system audio capture
> without screen capture.
> 
> Bug: 662499
> Change-Id: I0c612861e36644298f161de47810c6d37ffbf501
> Reviewed-on: https://chromium-review.googlesource.com/656597
> Commit-Queue: mark a. foltz <mfoltz@chromium.org>
> Reviewed-by: John Abd-El-Malek <jam@chromium.org>
> Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
> Reviewed-by: Guido Urdaneta <guidou@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#503491}

TBR=mfoltz@chromium.org,jam@chromium.org,sergeyu@chromium.org,guidou@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 662499
Change-Id: I0fa48ea0647a7c8d7186805aa097e20c505d1ba9
Reviewed-on: https://chromium-review.googlesource.com/691279
Commit-Queue: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505397}
[modify] https://crrev.com/979d466da3ae3d78c6a855fb6e1915be9c0e12fc/chrome/app/generated_resources.grd
[modify] https://crrev.com/979d466da3ae3d78c6a855fb6e1915be9c0e12fc/chrome/browser/media/webrtc/desktop_capture_access_handler.cc
[modify] https://crrev.com/979d466da3ae3d78c6a855fb6e1915be9c0e12fc/content/browser/renderer_host/media/media_stream_manager.cc
[modify] https://crrev.com/979d466da3ae3d78c6a855fb6e1915be9c0e12fc/content/public/browser/desktop_media_id.cc
[modify] https://crrev.com/979d466da3ae3d78c6a855fb6e1915be9c0e12fc/content/public/browser/desktop_media_id.h
[modify] https://crrev.com/979d466da3ae3d78c6a855fb6e1915be9c0e12fc/content/renderer/media/media_stream_dispatcher_unittest.cc

This broke Hangouts screenshare.  Will try to reland once I figure out what broke.

Mark, branch cut is today; is this moving M-66? Also should it be marked w/ the GetUserMedia component?

Setting this blocked on the mirror service refactor as that should give us greater flexibility in addressing this.