Fixing a bug where we increase read index with too few samples when the input is stereo. BUG=https://code.google.com/p/webrtc/issues/detail?id=714 R=andrew@webrtc.org Review URL: https://webrtc-codereview.appspot.com/1753004 git-svn-id: http://webrtc.googlecode.com/svn/trunk@4290 4adac7df-926f-26a2-2b94-8c16560cd09d