linux-aura: Make sure window stacking order is maintained.

Without the correct stacking order, GetWindowAtScreenPoint() returns the wrong window,
which can end up focusing the wrong window when a menu is dismissed by clicking on
overlapping parts of the windows.

BUG=317618
R=erg@chromium.org

Review URL: https://codereview.chromium.org/63613003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235453 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235453