Add dummy audio NACK APIs

R=pwestin@webrtc.org, turaj@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/1579006

git-svn-id: http://webrtc.googlecode.com/svn/trunk@4151 4adac7df-926f-26a2-2b94-8c16560cd09d
3 files changed