For my own reference, this is an example using a service in a RenderThread context: 

https://cs.chromium.org/chromium/src/content/renderer/gamepad_shared_memory_reader.cc?rcl=d6a383995a0e74853b81b22405229fe54b6b3728&l=25

And the service is registered in RenderProcessHostImpl::RegisterMojoInterfaces().

We also needs to add the equivalence of Plugin.AvailabilityStatus.WidevineCdm UMA in this new code path.

Assign to jrummell@ for load balance. It'll be great if we can land this early in M-65 timeframe (e.g. shortly after M-64 branch cut).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5d457dd1b8e515a59422d0dab4f18fd116e113dd

commit 5d457dd1b8e515a59422d0dab4f18fd116e113dd
Author: John Rummell <jrummell@chromium.org>
Date: Mon Dec 18 20:25:27 2017

media: Record whether a CDM supports persistent licenses or not

Updates CdmInfo to keep track if a CDM supports persistent licenses.

BUG= 764143 
TEST=updated content_unittest passes

Change-Id: I206daa59246a33c7a02e58b0356b617c175751a7
Reviewed-on: https://chromium-review.googlesource.com/826474
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Commit-Queue: John Rummell <jrummell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524785}
[modify] https://crrev.com/5d457dd1b8e515a59422d0dab4f18fd116e113dd/chrome/browser/component_updater/widevine_cdm_component_installer.cc
[modify] https://crrev.com/5d457dd1b8e515a59422d0dab4f18fd116e113dd/chrome/common/chrome_content_client.cc
[modify] https://crrev.com/5d457dd1b8e515a59422d0dab4f18fd116e113dd/content/browser/media/cdm_registry_impl_unittest.cc
[modify] https://crrev.com/5d457dd1b8e515a59422d0dab4f18fd116e113dd/content/common/media/cdm_info.cc
[modify] https://crrev.com/5d457dd1b8e515a59422d0dab4f18fd116e113dd/content/public/common/cdm_info.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/39082fe713310d01e66b71a632404726b70767eb

commit 39082fe713310d01e66b71a632404726b70767eb
Author: John Rummell <jrummell@chromium.org>
Date: Thu Dec 21 20:32:00 2017

media: Keep track of codecs as enums rather than strings

To ensure that the values for supported video codecs are valid,
keep track of them as enums.

BUG= 764143 
TEST=updated content_unittest passes

Change-Id: Ic984d580aa5967fa64b72cad3684b3c3ee9c5b9e
Reviewed-on: https://chromium-review.googlesource.com/838192
Commit-Queue: John Rummell <jrummell@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#525789}
[modify] https://crrev.com/39082fe713310d01e66b71a632404726b70767eb/chrome/browser/component_updater/widevine_cdm_component_installer.cc
[modify] https://crrev.com/39082fe713310d01e66b71a632404726b70767eb/chrome/common/chrome_content_client.cc
[modify] https://crrev.com/39082fe713310d01e66b71a632404726b70767eb/content/browser/media/cdm_registry_impl_unittest.cc
[modify] https://crrev.com/39082fe713310d01e66b71a632404726b70767eb/content/common/media/cdm_info.cc
[modify] https://crrev.com/39082fe713310d01e66b71a632404726b70767eb/content/public/common/cdm_info.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f4b8cba3f369a14dcf580aea1debef676590037d

commit f4b8cba3f369a14dcf580aea1debef676590037d
Author: John Rummell <jrummell@chromium.org>
Date: Mon Jan 08 20:35:35 2018

[media] Check for key system support using new mojo service KeySystemSupport

Instead of determining if the browser supports a key system by checking if a
particular plugin is registered, use the new mojo service KeySystemSupport to
query the browser and obtain information about the key system.

BUG= 764143 
TEST=new tests + modified EME browser_tests pass

Change-Id: Ied69bf2cc089f2620644f867cdcf7d7a2bce5ca0
Reviewed-on: https://chromium-review.googlesource.com/834764
Commit-Queue: John Rummell <jrummell@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#527741}
[modify] https://crrev.com/f4b8cba3f369a14dcf580aea1debef676590037d/chrome/browser/media/encrypted_media_browsertest.cc
[modify] https://crrev.com/f4b8cba3f369a14dcf580aea1debef676590037d/chrome/browser/media/encrypted_media_supported_types_browsertest.cc
[modify] https://crrev.com/f4b8cba3f369a14dcf580aea1debef676590037d/chrome/browser/media/library_cdm_test_helper.cc
[modify] https://crrev.com/f4b8cba3f369a14dcf580aea1debef676590037d/chrome/renderer/media/DEPS
[modify] https://crrev.com/f4b8cba3f369a14dcf580aea1debef676590037d/chrome/renderer/media/chrome_key_systems.cc
[modify] https://crrev.com/f4b8cba3f369a14dcf580aea1debef676590037d/content/browser/BUILD.gn
[add] https://crrev.com/f4b8cba3f369a14dcf580aea1debef676590037d/content/browser/media/key_system_support_impl.cc
[add] https://crrev.com/f4b8cba3f369a14dcf580aea1debef676590037d/content/browser/media/key_system_support_impl.h
[add] https://crrev.com/f4b8cba3f369a14dcf580aea1debef676590037d/content/browser/media/key_system_support_impl_unittest.cc
[modify] https://crrev.com/f4b8cba3f369a14dcf580aea1debef676590037d/content/browser/media/media_interface_proxy.cc
[modify] https://crrev.com/f4b8cba3f369a14dcf580aea1debef676590037d/content/browser/renderer_host/render_process_host_impl.cc
[modify] https://crrev.com/f4b8cba3f369a14dcf580aea1debef676590037d/content/public/app/mojo/content_browser_manifest.json
[modify] https://crrev.com/f4b8cba3f369a14dcf580aea1debef676590037d/content/public/renderer/BUILD.gn
[add] https://crrev.com/f4b8cba3f369a14dcf580aea1debef676590037d/content/public/renderer/key_system_support.cc
[add] https://crrev.com/f4b8cba3f369a14dcf580aea1debef676590037d/content/public/renderer/key_system_support.h
[modify] https://crrev.com/f4b8cba3f369a14dcf580aea1debef676590037d/content/test/BUILD.gn
[modify] https://crrev.com/f4b8cba3f369a14dcf580aea1debef676590037d/media/mojo/interfaces/BUILD.gn
[add] https://crrev.com/f4b8cba3f369a14dcf580aea1debef676590037d/media/mojo/interfaces/key_system_support.mojom

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/09c31164555c31f507c7f1940a88118204ef52df

commit 09c31164555c31f507c7f1940a88118204ef52df
Author: John Rummell <jrummell@chromium.org>
Date: Thu Jan 11 22:19:22 2018

Remove PluginInfoHost::IsInternalPluginAvailableForMimeType

Now that key system support is determined by KeySystemSupport, remove the
code for IsInternalPluginAvailableForMimeType and it's associated UMA.

BUG= 764143 
TEST=EME browser_tests pass

Change-Id: I256ad0685078877a1ba81fd6ba85526030f499b7
Reviewed-on: https://chromium-review.googlesource.com/855299
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: John Rummell <jrummell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#528780}
[modify] https://crrev.com/09c31164555c31f507c7f1940a88118204ef52df/chrome/browser/component_updater/widevine_cdm_component_installer.cc
[modify] https://crrev.com/09c31164555c31f507c7f1940a88118204ef52df/chrome/browser/plugins/plugin_info_host_impl.cc
[modify] https://crrev.com/09c31164555c31f507c7f1940a88118204ef52df/chrome/browser/plugins/plugin_info_host_impl.h
[modify] https://crrev.com/09c31164555c31f507c7f1940a88118204ef52df/chrome/common/chrome_content_client.cc
[modify] https://crrev.com/09c31164555c31f507c7f1940a88118204ef52df/chrome/common/plugin.mojom
[modify] https://crrev.com/09c31164555c31f507c7f1940a88118204ef52df/content/public/renderer/key_system_support.cc
[modify] https://crrev.com/09c31164555c31f507c7f1940a88118204ef52df/third_party/widevine/cdm/widevine_cdm_common.h
[modify] https://crrev.com/09c31164555c31f507c7f1940a88118204ef52df/tools/metrics/histograms/histograms.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fa6b26339b8df2b039dceefd73df802b2e6c1d2f

commit fa6b26339b8df2b039dceefd73df802b2e6c1d2f
Author: Xiaohan Wang <xhwang@chromium.org>
Date: Thu Jan 11 23:28:31 2018

media: Various fixes in EncryptedMediaSupportedTypes*Test

- Rename RegisterExternalClearKey() to RegisterClearKeyCdm(), as this
  function registers the Clear Key CDM to CdmRegister, not the key
  system itself.
- EncryptedMediaSupportedTypesExternalClearKeyNotEnabledTest is updated
  to use RegisterClearKeyCdm().
- EncryptedMediaSupportedTypesClearKeyCDMRegisteredWithWrongPathTest is
  updated using RegisterClearKeyCdm() as well.
- EncryptedMediaSupportedTypesWidevineCDMRegisteredWithWrongPathTest is
  removed as there's no way to actually register Widevine CDM using a
  wrong path.

BUG= 764143 

Change-Id: Icdf700ec5a4b85a0b2f0c4df3c3ef76fcc443184
Reviewed-on: https://chromium-review.googlesource.com/862295
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: John Rummell <jrummell@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#528813}
[modify] https://crrev.com/fa6b26339b8df2b039dceefd73df802b2e6c1d2f/chrome/browser/media/encrypted_media_browsertest.cc
[modify] https://crrev.com/fa6b26339b8df2b039dceefd73df802b2e6c1d2f/chrome/browser/media/encrypted_media_supported_types_browsertest.cc
[modify] https://crrev.com/fa6b26339b8df2b039dceefd73df802b2e6c1d2f/chrome/browser/media/library_cdm_test_helper.cc
[modify] https://crrev.com/fa6b26339b8df2b039dceefd73df802b2e6c1d2f/chrome/browser/media/library_cdm_test_helper.h
[modify] https://crrev.com/fa6b26339b8df2b039dceefd73df802b2e6c1d2f/chrome/common/chrome_content_client.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4dd77fe66cb9e8bda7b5f9d22bd8199372c769c9

commit 4dd77fe66cb9e8bda7b5f9d22bd8199372c769c9
Author: John Rummell <jrummell@chromium.org>
Date: Fri Jan 12 20:08:40 2018

Add UMA to report results from checking CDM availability for key system support

Now that the KeySystemSupport mojo interface is used to determine if a
key system is supported, keep track if the request succeeds or not.

This UMA replaces "Plugin.AvailabilityStatus.WidevineCdm" as the plugin
check is no longer done.

BUG= 764143 

Change-Id: I9a08b75cc533de0c1e6891879876a73df37c1e75
Reviewed-on: https://chromium-review.googlesource.com/855050
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Commit-Queue: John Rummell <jrummell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#529037}
[modify] https://crrev.com/4dd77fe66cb9e8bda7b5f9d22bd8199372c769c9/content/browser/media/key_system_support_impl.cc
[modify] https://crrev.com/4dd77fe66cb9e8bda7b5f9d22bd8199372c769c9/tools/metrics/histograms/histograms.xml