WebRtcVideoEngine is only ever accessed from one thread, so remove
the lock and replace it with ThreadChecker assertions.
Bug: None
Change-Id: I8c34eb6473f0ebaaaafe8a163c3f5d6f19074021
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/125240
Reviewed-by: Seth Hampson <shampson@webrtc.org>
Reviewed-by: Amit Hilbuch <amithi@webrtc.org>
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26957}
ef50b25690