Create unit tests for RtpDemuxer

1. Create unit tests for RtpDemuxer.
2. Add an RTC_DCHECK in RtpDemuxer that makes sure that the sink<->ssrc multimap does not allow multiple instances of the same association.

BUG=None

Review-Url: https://codereview.webrtc.org/2902823004
Cr-Original-Commit-Position: refs/heads/master@{#18357}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 760a076a5293b65d70c4f3cd171e148439b3608a
4 files changed