Add RateTracker for sent framerate and bitrate in SendStatisticsProxy. Store sent frame info in map to solve potential issue where sent framerate statistics could be incorrect. Bug: webrtc:8375 Change-Id: I4a6e3956013438a711b8c2e73a8cd90c52dd1210 Reviewed-on: https://webrtc-review.googlesource.com/7880 Reviewed-by: Erik Språng <sprang@webrtc.org> Commit-Queue: Åsa Persson <asapersson@webrtc.org> Cr-Commit-Position: refs/heads/master@{#20225}