Restore the void return type on WriteWavHeader.

Karl pointed out that the user can check the validity of the input
parameters with CheckWavParameters prior to calling.

TBR=kwiberg

Review URL: https://webrtc-codereview.appspot.com/23339004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7597 4adac7df-926f-26a2-2b94-8c16560cd09d