Delete unused and almost unused frame-related methods.

webrtc::VideoFrame::set_video_frame_buffer
webrtc::VideoFrame::ConvertNativeToI420Frame

cricket::WebRtcVideoFrame::InitToBlack

VideoFrameBuffer::data
VideoFrameBuffer::stride
VideoFrameBuffer::MutableData

TBR=tkchin@webrtc.org # Refactoring affecting RTCVideoFrame
BUG=webrtc:5682

Review-Url: https://codereview.webrtc.org/2065733003
Cr-Original-Commit-Position: refs/heads/master@{#13183}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 76270de4bc2dac188f10f805e6e2fb86693ef864
16 files changed