Fix mac video capture leak.

BUG=3878
R=mflodman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/38459004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7971 4adac7df-926f-26a2-2b94-8c16560cd09d