Add UMA logging for target audio bitrate

This CL logs the target audio bitrate to a UMA histogram called
WebRTC.Audio.TargetBitrateInKbps. It logs the rate when a codec is
created, and when the target is explicitly updated. Note that since
each codec implementation is free to change or ignore the target
value, there is no guarantee that the logged value will actually be
used as the target.

BUG=chromium:488124

Review URL: https://codereview.webrtc.org/1178053002

Cr-Commit-Position: refs/heads/master@{#9484}
6 files changed