This allows to protect ssrc_to_last_received_extended_high_seq_num_ member and
make calls to OnReceivedRtcpReceiverReport thread-safe without introducing new critical section.
Bug: webrtc:7735
Change-Id: Iee23bb780d07b0f906f1f8eeddde2b74cc0a2b89
Reviewed-on: https://chromium-review.googlesource.com/518130
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18540}