Permit mixing mono and stereo streams.

Add mixing tests based on older ones from the extended tests.

BUG=issue534
TEST=manual, voe_auto_test

Review URL: https://webrtc-codereview.appspot.com/576014

git-svn-id: http://webrtc.googlecode.com/svn/trunk@2265 4adac7df-926f-26a2-2b94-8c16560cd09d
5 files changed