This remove the use of VideoTrackRenderers within VideoTrack and instead all its sinks are passed to VideoSource.
That means that the source will handle all sinks and can (if the source implement it) handle different SinkWants for each sink.
The VideoBroadcaster is updated to produce black frames instead of as is today the deprecated VideoTrackRenderers.
BUG=webrtc:5426
R=nisse@webrtc.org, pthatcher@webrtc.org
Review URL: https://codereview.webrtc.org/1779063003 .
Cr-Commit-Position: refs/heads/master@{#12028}
