Add support for VP9 in webrtc::Call and video_loopback.

R=pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/31959004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7622 4adac7df-926f-26a2-2b94-8c16560cd09d