Making MockMixerAudioSource default to NiceMock.
Making MockMixerAudioSource default to NiceMock.

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 <2c708eb0a58047bf403399f50b3da33ffd2dd444@webrtc.org>
Commit-Queue: Sebastian Jansson <c24d1bc144eb7c11e553f59153fa17dba85ca83e@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#22245}
TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc