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