[Extensions] More bindings hardening

chrome/renderer/resources/extensions/automation_custom_bindings.js

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Custom bindings for the automation API.
 var AutomationNode = require('automationNode').AutomationNode;
 var AutomationRootNode = require('automationNode').AutomationRootNode;
 var automation = require('binding').Binding.create('automation');
 var automationInternal =
     require('binding').Binding.create('automationInternal').generate();
@@ skipping 306 matching lines @@
     privates(targetTree).impl.destroy();
     AutomationRootNode.destroy(id);
   } else {
     logging.WARNING('no targetTree to destroy');
   }
 
   // Destroy the native cache of the accessibility tree.
   DestroyAccessibilityTree(id);
 });
 
-exports.binding = automation.generate();
-
+var binding = automation.generate();
 // Add additional accessibility bindings not specified in the automation IDL.
 // Accessibility and automation share some APIs (see
 // ui/accessibility/ax_enums.idl).
 forEach(schema, function(k, v) {
-  exports.binding[k] = v;
+  binding[k] = v;
 });
+
+exports.$set('binding', binding);
+