Revert 252502 "Fixes DesktopScreenX11::GetWindowAtScreenPoint() ..."

Reason for revert: DesktopScreenX11Test.GetWindowAtScreenPoint started
failing on a few bots after this change.

> 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

TBR=pkotwicz@chromium.org

Review URL: https://codereview.chromium.org/174943003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252521 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252521