To support playing mono file with stereo codec as mixing with microphone capture

BUG=413
TEST=Manual test.
Review URL: https://webrtc-codereview.appspot.com/460004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@1953 4adac7df-926f-26a2-2b94-8c16560cd09d
4 files changed