MediaStreamTrack now have it's own readyState. This in preparation of
more standards compliance work: Track.stop should not any more stop the
source until all tracks using the same source is stopped.

BUG=357503

Review URL: https://codereview.chromium.org/219453002

git-svn-id: svn://svn.chromium.org/blink/trunk@170571 bbb929c8-8fbe-4397-9dbb-9b2b20218538