Adding a new ChangeLogger class to handle UMA logging of bitrates

This change introduces the sub-class ChangeLogger in AudioCodingModuleImpl. The class writes values to the named UMA histogram, but only if the value has changed since the last time (and always for the first call). This is to avoid the problem with audio codecs being registered but never used. Before this change, these codecs' bitrate was also logged, even though they were never used.

BUG=chromium:488124
R=kwiberg@webrtc.org

Review URL: https://codereview.webrtc.org/1203803004

Cr-Commit-Position: refs/heads/master@{#9506}
6 files changed