Move RTCI420Frame to webrtc/api/objc/RTCVideoFrame with minor style changes.

BUG=
R=tkchin@webrtc.org

Review URL: https://codereview.webrtc.org/1533193003 .

Patch from Jon Hjelle <hjon@andyet.net>.

Cr-Original-Commit-Position: refs/heads/master@{#11203}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 78234956985d837ebdc16b0fc5d6cea8de9192a2
5 files changed