Add API to swap stereo channels.

BUG=issue451
TEST=manually with voe_cmd_test, using stereo and mono codecs

Review URL: https://webrtc-codereview.appspot.com/519001

git-svn-id: http://webrtc.googlecode.com/svn/trunk@2106 4adac7df-926f-26a2-2b94-8c16560cd09d
7 files changed