Revert of Change VideoTrack implementation to invoke VideoTrackSourceInterface::AddOrUpdateSink on wt (patchset #2 id:20001 of https://codereview.webrtc.org/2964863002/ )

Reason for revert:
It breaks a downstream project.

Original issue's description:
> Change VideoTrack implementation to invoke VideoTrackSourceInterface::AddOrUpdateSink on the worker thread.
>
> Added documentation of thread expectations for video tracks and sources to the API.
>
> BUG=None
>
> Review-Url: https://codereview.webrtc.org/2964863002
> Cr-Commit-Position: refs/heads/master@{#18938}
> Committed: https://chromium.googlesource.com/external/webrtc/+/f1377f722271b4b5a090d4dba10201031d31f20d

TBR=deadbeef@webrtc.org,noahric@chromium.org,yujo@chromium.org,perkj@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=None

Review-Url: https://codereview.webrtc.org/2979493003
Cr-Commit-Position: refs/heads/master@{#18942}
13 files changed
tree: 1cc96537a96d6577bf312713b150ae1d05d44ce7
  1. build_overrides/
  2. data/
  3. infra/
  4. resources/
  5. tools_webrtc/
  6. webrtc/
  7. .clang-format
  8. .git-blame-ignore-revs
  9. .gitignore
  10. .gn
  11. AUTHORS
  12. BUILD.gn
  13. check_root_dir.py
  14. cleanup_links.py
  15. CODE_OF_CONDUCT.md
  16. codereview.settings
  17. DEPS
  18. LICENSE
  19. license_template.txt
  20. LICENSE_THIRD_PARTY
  21. OWNERS
  22. PATENTS
  23. PRESUBMIT.py
  24. pylintrc
  25. README.md
  26. WATCHLISTS
README.md

WebRTC is a free, open software project that provides browsers and mobile applications with Real-Time Communications (RTC) capabilities via simple APIs. The WebRTC components have been optimized to best serve this purpose.

Our mission: To enable rich, high-quality RTC applications to be developed for the browser, mobile platforms, and IoT devices, and allow them all to communicate via a common set of protocols.

The WebRTC initiative is a project supported by Google, Mozilla and Opera, amongst others.

Development

See http://www.webrtc.org/native-code/development for instructions on how to get started developing with the native code.

More info