VideoTrackAdapter: Add passing frames monitor, notify MSVCS -> MSVTrack(s)
MediaStreamVideoSource owns a VideoTrackAdapter, that sees the
frames passing by (possibly adapting frame rate and resolution).
This CL extends this VTA to monitor passing frames. Every time the
monitoring wakes up, it notifies the MediaStreamVideoSource of
the muted state via SetMutedState. This Class has no state, simply
updates all registered MediaStreamVideoTracks, who have a
muted state.
Later CLs will further connect the MSVT::SetMutedState() to
ping WebMediaStreamTrack etc.
BUG=389159
Review URL: https://codereview.chromium.org/
366243003
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@283285 0039d316-1c4b-4281-b951-d872f2087c98