Bug 919683 - Fix regression from bug 880930. r=past

webapprt/content/dbg-webapp-actors.js

@@ -52,7 +52,7 @@ WebappTabList.prototype = Object.create(BrowserTabList.prototype);
 WebappTabList.prototype.constructor = WebappTabList;
 
 WebappTabList.prototype.getList = function() {
-  let topXULWindow = windowMediator.getMostRecentWindow(this._windowType);
+  let topXULWindow = Services.wm.getMostRecentWindow(this._windowType);
 
   // As a sanity check, make sure all the actors presently in our map get
   // picked up when we iterate over all windows.