Implement getAssociatedSink method of webrtcAudioPrivate API.

BUG=298158

Review URL: https://codereview.chromium.org/32573004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230694 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230694