Update expectation of OneBitrateObserverTwoRtcpObservers test:
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-Original-Commit-Position: refs/heads/master@{#18536}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 7f8369aa3ff404257ec66526ebbb3419602b0056