Refactor AudioUnit code into its own class.

BUG=
R=haysc@webrtc.org, henrika@webrtc.org

Review URL: https://codereview.webrtc.org/1809343002 .

Cr-Original-Commit-Position: refs/heads/master@{#12056}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 1300caa3fea840572614772f96755ce7f4080ef6
7 files changed