linux_aura: Implement the rest of DesktopScreenX11.

When trying to call GetDisplayNearestWindow(), we should just query the
bounds of the DRWHX11 instead of either going through the
aura::Window::GetBoundsInScreen() machinery or going to the X server.

Also fixes a small regression in tracking open_windows() caused by a bad
merge.

BUG=287972
R=sadrul@chromium.org

Review URL: https://codereview.chromium.org/24513004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225765 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225765