This is done by changing the RateStatistics so that it resets its window when the accumulator is empty. It also keeps a dynamic window, so that the rates computed before a full window worth of data has been received will be computed over a smaller window. This means that the rate will be closer to the true rate, but with a higher variance.
BUG=webrtc:5773
R=perkj@webrtc.org, sprang@webrtc.org
Review URL: https://codereview.webrtc.org/1908893003 .
Cr-Commit-Position: refs/heads/master@{#12470}
