Stylizing AudioConferenceMixer.

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}
8 files changed