Support VP8 encoder settings in VideoSendStream.

Stop-gap solution to support VP8 codec settings in the new API until
encoder settings can be passed on to the VideoEncoder without requiring
explicit support for the codec.

BUG=3424
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/17929004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6650 4adac7df-926f-26a2-2b94-8c16560cd09d