This eliminates a lot of warnings when running AudioMixer unittests.
Bug: None
Change-Id: I978e859e8904bb382843a9c23a35ace1884aa4b4
Reviewed-on: https://webrtc-review.googlesource.com/58981
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22245}