Move output_mixer_unittest.cc to utility_unittest.cc.

This reflects a move of the tested code in:
https://webrtc-codereview.appspot.com/11019005/

TBR=xians

Review URL: https://webrtc-codereview.appspot.com/11449004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@5866 4adac7df-926f-26a2-2b94-8c16560cd09d
2 files changed