WIP: Isolate the libjingle dependency of WebRtcLocalAudioTrack into an adapter class.

This CL is taking over from https://codereview.chromium.org/123943003/ which creates an adapter for the local audio track and hide all the libjingle webrtc interfaces/objects into the adapter.

BUG=325675
TEST=content_unittests and content_browsertets, nothing breaks.
R=jamesr@chromium.org, joi@chromium.org

Review URL: https://codereview.chromium.org/151503004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248712 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248712