This enabled us to be able to remove VideoTrack::GetSink and RemoteVideoCapturer. Since video frames from the decoder is delivered on a media engine internal thread, VideoBroadCaster must be made thread safe. BUG=webrtc:5426 R=deadbeef@webrtc.org, pthatcher@webrtc.org Review URL: https://codereview.webrtc.org/1765423005 . Cr-Commit-Position: refs/heads/master@{#11944}