This is the second of ~three CLs for adding the audio component of
MediaStreamRecording. These CLs will be:
1) Add AudioTrackRecorder, a MediaStreamAudioSink which will be owned by MediaRecorderHandler
2) Update WebmMuxer to mux Opus output of AudioTrackRecorder
3) Update MediaRecorderHandler to use AudioTrackRecorder

Also refactors some opus header-related constants into a common location.

BUG=528519

Review URL: https://codereview.chromium.org/1414793002

Cr-Commit-Position: refs/heads/master@{#358489}