Hey, I'm a newbie to this project and I'm interested in trying this out.

Do I understand correctly that the objective is to change the prototype, implementation and documentation of `VideoCaptureController::ReturnBuffer()` to remove arg `const gpu::SyncToken& sync_token`?

Yes, the objective is as you described, to remove gpu::SyncToken references from these methods. However it exists in many places:
- All refs in content:VideoCaptureImpl that handles on renderer process.
- Ref in ReleaseBuffer() video_capture.mojom that is for browser-renderer process communication.
- VideoCaptureHost::ReleaseBuffer() that handles message on browser side
- All refs in VideoCaptureController that runs on browser process.
You can tackle these in multiple changelists if you find the scope to be too big. Please let me know if you have any more questions.

Yes, looks like this change has a tendency to creep around the codebase. So I'll start with a first patch that will keep the (ignored) arg in `OnClientBufferFinished` and then we'll see.

Attached a patch here: https://codereview.chromium.org/2442193002 . I may have done something wrong because I don't see any link between the patch and the issue.

#5: there won't any explicit link from here to the CL
until this is landed. The CL has a BUG= line pointing here.

Assigning the bug to emircan@ as proxy to d.o.teller@

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/214a2c9b14523bebc5ccef00e8df54099e3a619a

commit 214a2c9b14523bebc5ccef00e8df54099e3a619a
Author: emircan <emircan@chromium.org>
Date: Tue Jan 03 22:26:58 2017

Removing gpu::SyncToken usage from video capture pipeline

This CL is patched from https://codereview.chromium.org/2442193002/. All
credit goes to D.O.Teller@gmail.com.

The use of gpu::SyncToken has been added for the sake of capture into
textures, but is now useless. This patch removes all uses up to
`OnClientBufferFinished` (excluded).

BUG= 653994 

Review-Url: https://codereview.chromium.org/2583883003
Cr-Commit-Position: refs/heads/master@{#441223}

[modify] https://crrev.com/214a2c9b14523bebc5ccef00e8df54099e3a619a/content/browser/renderer_host/media/video_capture_controller.cc
[modify] https://crrev.com/214a2c9b14523bebc5ccef00e8df54099e3a619a/content/browser/renderer_host/media/video_capture_controller.h
[modify] https://crrev.com/214a2c9b14523bebc5ccef00e8df54099e3a619a/content/browser/renderer_host/media/video_capture_controller_unittest.cc
[modify] https://crrev.com/214a2c9b14523bebc5ccef00e8df54099e3a619a/content/browser/renderer_host/media/video_capture_host.cc
[modify] https://crrev.com/214a2c9b14523bebc5ccef00e8df54099e3a619a/content/browser/renderer_host/media/video_capture_host.h
[modify] https://crrev.com/214a2c9b14523bebc5ccef00e8df54099e3a619a/content/common/video_capture.mojom
[modify] https://crrev.com/214a2c9b14523bebc5ccef00e8df54099e3a619a/content/renderer/media/video_capture_impl.cc
[modify] https://crrev.com/214a2c9b14523bebc5ccef00e8df54099e3a619a/content/renderer/media/video_capture_impl.h
[modify] https://crrev.com/214a2c9b14523bebc5ccef00e8df54099e3a619a/content/renderer/media/video_capture_impl_manager_unittest.cc
[modify] https://crrev.com/214a2c9b14523bebc5ccef00e8df54099e3a619a/content/renderer/media/video_capture_impl_unittest.cc

BTW, there is still a TODO referencing this bug in content/common/BUILD.gn.

Removing it here: https://codereview.chromium.org/2866743002/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0894550fc7a4d5bd11d5b9992383a701b6177428

commit 0894550fc7a4d5bd11d5b9992383a701b6177428
Author: emircan <emircan@chromium.org>
Date: Fri May 05 23:44:47 2017

Remove dependency after gpu::SyncToken is no longer used in capture

BUG= 653994 

Review-Url: https://codereview.chromium.org/2866743002
Cr-Commit-Position: refs/heads/master@{#469807}

[modify] https://crrev.com/0894550fc7a4d5bd11d5b9992383a701b6177428/content/common/BUILD.gn