Use different media ssrcs for different RtcpBandwidthObservers
Bug: None
Change-Id: I1733ddfa5dcd378b700e31fd805d8930ec69064f
Reviewed-on: https://chromium-review.googlesource.com/517798
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18536}