Adding missing break in media_file_utility.cc.

There has been no reports of problems, but adding this to get it correct.

Review URL: https://webrtc-codereview.appspot.com/19599004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6322 4adac7df-926f-26a2-2b94-8c16560cd09d