(And similarly for U and V). Also change video_frame_buffer method to
return a const ref to a scoped_ref_ptr.
This cl is analogous to https://codereview.webrtc.org/1900673002/,
which delete corresponding methods in webrtc::VideoFrame.
BUG=webrtc:5682
Review URL: https://codereview.webrtc.org/1901973002
Cr-Commit-Position: refs/heads/master@{#12492}