RTCVideoFrame is an ObjectiveC version of webrtc::VideoFrame, but it currently contains some extra logic beyond that. We want RTCVideoFrame to be as simple as possible, i.e. just a container with no extra state, so we can use it as input to RTCVideoSource without complicating the interface for consumers. BUG=webrtc:7177 NOTRY=True Review-Url: https://codereview.webrtc.org/2695203004 Cr-Commit-Position: refs/heads/master@{#16740}