layout: Flush region update in showOverview

We cannot wait for the queued update region to fire when
xdnd is being used because a wrong input shape can result
into a xdnd leave event when the user moves the pointer fast.

https://bugzilla.gnome.org/show_bug.cgi?id=708887

js/ui/layout.js

@@ -251,7 +251,7 @@ const LayoutManager = new Lang.Class({
 
         this._inOverview = true;
         this._updateVisibility();
-        this._queueUpdateRegions();
+        this._updateRegions();
     },
 
     hideOverview: function() {