For simulcast VideoSendStreams there are more than one entry in the SSRC
table causing RTCP to currently be delivered more than once per stream.
This messes up unique NACK stats as all NACK requests sent to such a
sender will be delivered multiple times and not look unique.
BUG=4544
R=asapersson@webrtc.org
Review URL: https://webrtc-codereview.appspot.com/50709004
Cr-Commit-Position: refs/heads/master@{#8998}
74b9769e4e