RtcpTransceiver name reserved for thread-safe version that planned to be wrapper of the RtcpTransceiverImpl BUG=webrtc:8239 Change-Id: If8a3092eb1b8e4175e3efd23b52e1043cdabf19f Reviewed-on: https://webrtc-review.googlesource.com/7920 Commit-Queue: Danil Chapovalov <danilchap@webrtc.org> Reviewed-by: Niels Moller <nisse@webrtc.org> Cr-Commit-Position: refs/heads/master@{#20414}