Issue 4139 - Fixed memory leak due to bug in previous patch

lib/child/elemHide.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2016 Eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 121 matching lines @@
   status: Cr.NS_OK,
 
   _getResponse: notImplemented,
 
   _checkSecurity: function()
   {
     if (!this.loadInfo.triggeringPrincipal.equals(Utils.systemPrincipal))
       throw Cr.NS_ERROR_FAILURE;
   },
 
-  asyncOpen: function()
+  asyncOpen: function(listener, context)
   {
     Promise.resolve(this._getResponse()).then(data =>
     {
       let stream = Cc["@mozilla.org/io/string-input-stream;1"]
                      .createInstance(Ci.nsIStringInputStream);
       stream.setData(data, data.length);
 
       try
       {
         listener.onStartRequest(this, context);
@@ skipping 133 matching lines @@
     return new Promise((resolve, reject) =>
     {
       let window = Utils.getRequestWindow(this);
       shouldAllowAsync(window, window.document, "ELEMHIDE", this.key, allow =>
       {
         resolve(allow ? allowXBL : hideXBL);
       });
     });
   }
 };