Cleaning AudioConferenceMixer APIs to match Chromium style guide.
Main changes:
1. change all mutable references to pointers
2. add const to all non-mutable references
3. add const to as many methods as possible
BUG=
R=andrew@webrtc.org
Review URL: https://codereview.webrtc.org/1311733003 .
Cr-Commit-Position: refs/heads/master@{#9821}
0f4b3731c3