layout: Fix opening the overview for xdnd

In the (no barriers) fallback case handleDragOver has somehow
ended up being turned into a nop and thus breaking xdnd
overview opening.

Fix that by calling _toggleOverview() when a xdnd source triggers
it.

https://bugzilla.gnome.org/show_bug.cgi?id=696447

js/ui/layout.js

@@ -1200,6 +1200,8 @@ const HotCorner = new Lang.Class({
         if (source != Main.xdndHandler)
             return DND.DragMotionResult.CONTINUE;
 
+        this._toggleOverview();
+
         return DND.DragMotionResult.CONTINUE;
     },