Move the webaudio source provider to the track.

The WebAudio source provider should not belong to the source since it is connected to a media stream track.
Also, we should use the generic sink interface for all the clients of the track, and do not make webaudio special.




BUG=314639
TEST=content_unittests --gest_filter="*WebRtc*"

Review URL: https://codereview.chromium.org/48993002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235202 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235202