Fix multi-monitor support in the screen capturer for Mac.

This feature was broken in r5471.

BUG=361919
R=jiayl@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/12109004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@5937 4adac7df-926f-26a2-2b94-8c16560cd09d