Fix mac video_render implementation on cocoa.

Hit this while playing around with all compile possibilities for issue 3770.

BUG=3770
TBR=mflodman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/23629004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7166 4adac7df-926f-26a2-2b94-8c16560cd09d