Update I420Buffer to new VideoFrameBuffer interface

This is a follow-up cleanup for CL https://codereview.webrtc.org/2847383002/.

BUG=webrtc:7632
TBR=stefan

Review-Url: https://codereview.webrtc.org/2906053002
Cr-Original-Commit-Position: refs/heads/master@{#18388}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 3f075498a3a3d7426deded720e21a6f77d1cabef
20 files changed