Reason for revert:
Relanding
Original issue's description:
> Revert of SSRC and RSID may only refer to one sink each in RtpDemuxer (patchset #15 id:280001 of https://codereview.webrtc.org/2968693002/ )
>
> Reason for revert:
> Some internal tests keep failing after this change. Try to fix it by reverting it. Will reland it if this isn't the root cause.
>
> Original issue's description:
> > SSRC and RSID may only refer to one sink each in RtpDemuxer
> >
> > RTP demuxing should only match RTP packets with one sink.
> >
> > BUG=webrtc:7135
> >
> > Review-Url: https://codereview.webrtc.org/2968693002
> > Cr-Commit-Position: refs/heads/master@{#19233}
> > Committed: 7b7e06fd23
>
> TBR=nisse@webrtc.org,danilchap@webrtc.org,perkj@webrtc.org,stefan@webrtc.org,holmer@google.com,deadbeef@webrtc.org,pthatcher@webrtc.org,steveanton@webrtc.org,eladalon@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:7135
>
> Review-Url: https://codereview.webrtc.org/2993633002
> Cr-Commit-Position: refs/heads/master@{#19239}
> Committed: 59b603fbedTBR=nisse@webrtc.org,danilchap@webrtc.org,perkj@webrtc.org,stefan@webrtc.org,holmer@google.com,deadbeef@webrtc.org,pthatcher@webrtc.org,steveanton@webrtc.org,zhihuang@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:7135
Review-Url: https://codereview.webrtc.org/2993053002
Cr-Commit-Position: refs/heads/master@{#19248}
5daecca41b