Added VideoTrackInterface::GetSink method, for use by VideoRtpReceiver. This lets us delete FrameInput. I realized this change doesn't depend on VideoSinkInterface changes, so this is a more standalone version of cl https://codereview.webrtc.org/1664773002/ BUG=webrtc:5426 Review URL: https://codereview.webrtc.org/1660103003 Cr-Commit-Position: refs/heads/master@{#11498}