Currently, parameters are periodically updated, but the TargetBitrate
message is only sent if a new bitrate is set. It should be sent
periodically to indicate the signaled bitrate is valid and to prevent
stale values due to loss of RTCP packets.
BUG=webrtc:6897
Review-Url: https://codereview.webrtc.org/2616393003
Cr-Commit-Position: refs/heads/master@{#16096}