This allows clients to enable Receiver reference time reports via
PeerConnection.
RRTR is not enabled by default but can be added to SDP string.
Bug: webrtc:9108
Change-Id: I851f0d65152875bf115553a851b839f83e3d241e
Reviewed-on: https://webrtc-review.googlesource.com/66861
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22767}