Fix memory leak in webrtc::MouseCursorMonitorMac

BUG=3815
R=sergeyu@chromium.org

Review URL: https://webrtc-codereview.appspot.com/24579004

Patch from Vicken Simonian <vsimon@gmail.com>.

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7187 4adac7df-926f-26a2-2b94-8c16560cd09d