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When multiple getUserMedia and stop() requests are active concurrently, 
getUserMedia() sometimes fails with TrackStartError.
The reason is that the underlying video capture subsystem does not keep
a count of how many sources have opened a video device.
It can happen that a stop request stops video capture while a 
getUserMedia() request is being processed, with the result being that
the video device is closed before getUserMedia() starts the new video
track, resulting in an error.

One solution to this problem is to make sure that stop requests and
getUserMedia() requests cannot run concurrently, which is the approach
followed by this CL.

This CL basically moves handling of MediaStramTrack.stop() from 
MediaStreamCenter to UserMediaClient, and puts stop() requests in the
same queue used for getUserMedia() and applyConstraints() requests.


Bug: 778039
Change-Id: Id7e0ee38e40265fd599dc6dd0712f0b667d2e1a5
Reviewed-on: https://chromium-review.googlesource.com/751981Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515494}