Add missing DesktopConfigurationMonitor Unlock in webrtc::ScreenCapturerMac

BUG=3837
R=jiayl@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/30469004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7291 4adac7df-926f-26a2-2b94-8c16560cd09d