Clear bitrate stats for unused SSRCs.

Prevents bug where transmitted bitrate was reported as higher than what
was actually sent, since unused RTP modules weren't updated to say that
they sent zero.

BUG=
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/49979004

Cr-Commit-Position: refs/heads/master@{#9192}
4 files changed