Enable external encoders with internal picture source.

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/webrtc@3344 4adac7df-926f-26a2-2b94-8c16560cd09d
7 files changed