Cast: deliver video frames on the IO thread
This is the last change to deliver video frames to Cast streaming API
on the IO thread. It will no longer be blocked by Javascript events.
This needs a change in the MediaStreamVideoSink interface. Also getting
rid of the interface of MediaStreamVideoTrack that delivers frames on
the render thread. All clients now handle the thread hopping directly.
BUG=335327, 371775
Review URL: https://codereview.chromium.org/
264363005
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@269616 0039d316-1c4b-4281-b951-d872f2087c98