Remove unused implementations in cricket::VideoFrame

This CL moves dummy implementations from cricket::VideoFrame to NullVideoFrame instead.

R=guoweis@webrtc.org, perkj@webrtc.org, pthatcher@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/50409004

Cr-Commit-Position: refs/heads/master@{#8873}
3 files changed