This patch tries to only change the interface to VideoTrack, with minimal changes to the implementation. Some points worth noting: VideoTrackRenderers should ultimately be deleted, but it is kept for now since we need an object implementing webrtc::VideoRenderer, and that shouldn't be VideoTrack. BUG=webrtc:5426 TBR=glaznev@webrtc.org // please look at examples Review URL: https://codereview.webrtc.org/1684423002 Cr-Commit-Position: refs/heads/master@{#11775}