CL enables registering of external encoder with internal picture source on API by adding simple passthrough parameter that is already supported within video engine. Review URL: https://webrtc-codereview.appspot.com/1006006 git-svn-id: http://webrtc.googlecode.com/svn/trunk@3344 4adac7df-926f-26a2-2b94-8c16560cd09d