Work around data race in TransmitMixer.

BUG=chromium:389098

Review URL: https://codereview.webrtc.org/1466353003

Cr-Original-Commit-Position: refs/heads/master@{#10772}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 302c978c921c2c63e1ab2414f0db03d19d5bfc60
1 file changed