This CL makes sure the real VideoTrackSourceInterface implementation is
destroyed on the signaling thread and marshals all method calls to the
signaling thread. This is done using VideoTrackSourceProxy.
Bug: webrtc:7767
Change-Id: Iba3b67bb32a684ba289bc8b9981585ea58084359
Reviewed-on: https://chromium-review.googlesource.com/526634
Commit-Queue: Magnus Jedvert <magjed@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18476}