Use XErrorTrap in MouseCursorMonitorX11 to catch the error if the shared window has been closed.

BUG=crbug/374457
R=sergeyu@chromium.org

Review URL: https://webrtc-codereview.appspot.com/13599004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6359 4adac7df-926f-26a2-2b94-8c16560cd09d