Allow parsing empty RTCP TargetBitrate messages, but stop sending them.

Also, add ToString() convenience method to the target bitrate struct. Super useful when doing printf debugging :)

BUG=webrtc:7858

Review-Url: https://codereview.webrtc.org/2947633003
Cr-Original-Commit-Position: refs/heads/master@{#18717}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: d0fc37a8841ed70b5338ce80ac9c3e8ac139a310
9 files changed