This change is due to an incorrect understanding of the threading
model in Chrome. The new AudioMixer has a thread checker to ensure
that mixing is always done from a single thread. Mixing is done on the
Audio Output Thread. When run in Chrome, it can change. Even if the thread
changes, there is never more than one audio thread, and mixing is done
sequentially.
The threading checks and variable access checks are replaced with
rtc::RaceChecker counterparts.
NOTRY=True
BUG=webrtc:6346
Review-Url: https://codereview.webrtc.org/2437913003
Cr-Commit-Position: refs/heads/master@{#14712}