This new VideoFrame class closesly matches the C++ webrtc::VideoFrame
and webrtc::VideoFrameBuffer classes. It's supposed to replace the
existing VideoRenderer.I420Frame. The purpose is to clean up the code
and support more frame formats.
BUG=webrtc:7749
Review-Url: https://codereview.webrtc.org/2915083002
Cr-Commit-Position: refs/heads/master@{#18404}