Previously BirateProber was calculating delay between probes based on the size of the previous probe. Because of that the actual sent bitrate can deviate greatly from the target value. With this change it uses total number of bytes in the cluster to estimate delay before each probe. BUG=webrtc:6952 Review-Url: https://codereview.webrtc.org/2613543003 Cr-Commit-Position: refs/heads/master@{#15971}