Reason for revert:
Multiple definitions of webrtc::MockMixerParticipant::MockMixerParticipant() during linking of modules_unittests. Please investigate and resubmit.
Original issue's description:
> Rewrote UpdateToMix in the audio mixer.
>
> The new version is much shorter than the old one, and hopefully easier
> to read. This is part of the effort to rewrite the old mixer.
>
> Committed: https://crrev.com/2942e240f4a985752714dac18c141064c97696d4
> Cr-Commit-Position: refs/heads/master@{#13568}
TBR=ossu@webrtc.org,ivoc@webrtc.org,aleloi@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
Review-Url: https://codereview.webrtc.org/2195633002
Cr-Commit-Position: refs/heads/master@{#13569}
ea4c141ffa