Add (safe) uint32_t cast to fix Win64 build.

TBR=kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/36539004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7916 4adac7df-926f-26a2-2b94-8c16560cd09d