Skip encoding in fake VP8 encoder.

Broke memcheck, FakeEncoder::Encode doesn't produce valid VP8 frames.

BUG=3424
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/13899004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6652 4adac7df-926f-26a2-2b94-8c16560cd09d