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This changes monitoring of video frames delivered to MediaStreamVideoSource to be started when the source is started and stopped when the source is stopped rather than depend on
that the source object is destroyed since destruction is dependent on when blink runs garbage collection.

It removes the unused MediaStreamVideoTrack::SetMuted and GetMuted.

It reduces the need for a thread hop between the io-thread and the render thread for each monitor intervall by only trigger the monitoring callback on if a change has occured.

It reduces the test time of media_stream_video_source_unittest.cc from more than 7s to around 200ms.

BUG= 404106

Review URL: https://codereview.chromium.org/509873002

Cr-Commit-Position: refs/heads/master@{#292853}