Move default-recv-channels to a separate class.

BUG=1788,3099
R=pthatcher@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/20119004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6879 4adac7df-926f-26a2-2b94-8c16560cd09d