grabHelper: Treat the current grabbed actor as a grabbed actor

This should be obvious, but I guess it wasn't necessary for the
message tray case.

js/ui/grabHelper.js

@@ -77,9 +77,12 @@ const GrabHelper = new Lang.Class({
     },
 
     _isWithinGrabbedActor: function(actor) {
+       let currentActor = this.currentGrab.actor;
         while (actor) {
             if (this._actors.indexOf(actor) != -1)
                 return true;
+            if (actor == currentActor)
+                return true;
             actor = actor.get_parent();
         }
         return false;