Allow same src and dst in InputAudioFile::DuplicateInterleaved

This change allows the input and output to the static method
InputAudioFile::DuplicateInterleaved to be the same array. That is,
in-place manipulation is now possible. A unit test is also added.

R=tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/20239004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7008 4adac7df-926f-26a2-2b94-8c16560cd09d