Add NACK rate throttling for audio channels.

Not really used for audio today (already in place for video), but should
still function anyway.

BUG=
R=henrika@webrtc.org, minyue@webrtc.org, stefan@webrtc.org

Review URL: https://codereview.webrtc.org/2181383002 .

Cr-Original-Commit-Position: refs/heads/master@{#13571}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 737336d37afb29e20ea406b875c74fbd9f95f1b8
18 files changed