Adding an empty constructor implementation to the AudioSink class

Turns out it was needed.

TBR=kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/15849004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6493 4adac7df-926f-26a2-2b94-8c16560cd09d