VoE Changes to enable dual_streaming.

TEST=added new unit-test

This CL depends on issue 933015 http://webrtc-codereview.appspot.com/933015/
which is under review. Should be committed after issue 933015 is committed.
Review URL: https://webrtc-codereview.appspot.com/970005

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@3231 4adac7df-926f-26a2-2b94-8c16560cd09d
8 files changed