Remove CriticalSectionWrapper from audio conference mixer.

BUG=webrtc:7035

Review-Url: https://codereview.webrtc.org/2772573006
Cr-Original-Commit-Position: refs/heads/master@{#17381}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 90947052d5f50ddd5a929275a41dda3635ea0504
2 files changed