Implement webrtcAudioPrivate API, except for getAssociatedSink.

Add an extension browser test for all of the implemented methods and the event.

Extend AudioOutputController tests to verify that SwitchOutputDevice works correctly in the face of diverted streams.

BUG=298158

Review URL: https://codereview.chromium.org/26839002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230398 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230398