windowManager: Fix a warning

The destroy effect currently relies on a monkey-patched property
added from the map effect. However on X11 it is possible that we
did no map animation for a window that is destroyed when the shell
was restarted or had taken over from another WM. Just use the real
MetaWindow property to avoid a warning in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=781471

js/ui/windowManager.js

@@ -1614,7 +1614,7 @@ const WindowManager = new Lang.Class({
             return;
         }
 
-        switch (actor._windowType) {
+        switch (actor.meta_window.window_type) {
         case Meta.WindowType.NORMAL:
             actor.set_pivot_point(0.5, 0.5);
             this._destroying.push(actor);