Return expected type from handleDragOver for Activities button actors

Currently they return 'undefined' instead of something meaningful,
e.g. DND.DragMotionResult.CONTINUE. This was unnoticed because none
of the ancestors of the Activities button actors do any drag handling.
The only visible issue are JS errors generated when dragging, for example,
a window thumbnail over the button, because the cursor cannot be set.

https://bugzilla.gnome.org/show_bug.cgi?id=669921

js/ui/layout.js

@@ -8,6 +8,7 @@ const Shell = imports.gi.Shell;
 const Signals = imports.signals;
 const St = imports.gi.St;
 
+const DND = imports.ui.dnd;
 const Main = imports.ui.main;
 const Params = imports.misc.params;
 const ScreenSaver = imports.misc.screenSaver;
@@ -490,13 +491,15 @@ const HotCorner = new Lang.Class({
 
     handleDragOver: function(source, actor, x, y, time) {
         if (source != Main.xdndHandler)
-            return;
+            return DND.DragMotionResult.CONTINUE;
 
         if (!Main.overview.visible && !Main.overview.animationInProgress) {
             this.rippleAnimation();
             Main.overview.showTemporarily();
             Main.overview.beginItemDrag(actor);
         }
+
+        return DND.DragMotionResult.CONTINUE;
     },
 
     _onCornerEntered : function() {