This includes if RTCP is received, but the number of packets received by the
other end hasn't increased.
Further, if no RTCP is received for more than 3 feedback intervals (3 seconds)
we start reducing the estimate by 20%. This is put under an experiment.
BUG=webrtc:6238
R=terelius@webrtc.org
Review URL: https://codereview.webrtc.org/2262213002 .
Cr-Commit-Position: refs/heads/master@{#14306}