Make an AudioEncoder subclass for comfort noise

BUG=3926
R=bjornv@webrtc.org, kjellander@webrtc.org, kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/31129004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@7857 4adac7df-926f-26a2-2b94-8c16560cd09d
9 files changed