This adds the RemoteEstimate rtcp packet and wires it up to GoogCC where it's used to improve congestion controller behavior. The functionality is negotiated using SDP. It's added with a field trial that allow disabling the functionality in case there's any issues. Bug: webrtc:10742 Change-Id: I1ea8e4216a27cd2b00505c99b42d1e38726256c8 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/146602 Reviewed-by: Stefan Holmer <stefan@webrtc.org> Reviewed-by: Erik Språng <sprang@webrtc.org> Commit-Queue: Sebastian Jansson <srte@webrtc.org> Cr-Commit-Position: refs/heads/master@{#28654}