Fixes DesktopScreenX11::GetWindowAtScreenPoint() so that NULL is returned if a chrome browser window is occluded by a non Chrome app at the passed in point

BUG=339587
TEST=Manual, see bug

Review URL: https://codereview.chromium.org/173193006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252502 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252502