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This CL adds new methods to support restarting a MediaStreamVideoSource.
By default, sources continue to be nonrestartable, but the
MediaStreamVideoCapturerSource does implement the restart capability.

This is a preparatory CL to support MediaStreamTrack.applyConstraints(),
which requires the ability to change the video format of a video source.

Bug: 763319
Change-Id: I91b868326f1fcc52a8146612e4cd0429774f82e1
Reviewed-on: https://chromium-review.googlesource.com/675687
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504344}