Adding owner access from WebMediaStreamTrack::ExtraData
Like WebMediaStreamSource having access to the owner of the ExtraData
object simplifies the chromium code significantly.
Also taking the opportunity to clean up the ExtraData code in
MediaStreamSource after the last of the non-standard code added for
WebAudio has been refactored away.

Review URL: https://codereview.chromium.org/133733004

git-svn-id: svn://svn.chromium.org/blink/trunk@165054 bbb929c8-8fbe-4397-9dbb-9b2b20218538