Reland "Adding unittests to AudioConferenceMixer."

Previous code review, see
https://codereview.chromium.org/1257583011/

Did not pass Mac bots since vector(size_t n) needs copy ctor before C++11.

BUG=

Review URL: https://codereview.webrtc.org/1303273004

Cr-Commit-Position: refs/heads/master@{#9851}
2 files changed