[Extensions] More bindings hardening

chrome/renderer/resources/extensions/page_capture_custom_bindings.js

 // Copyright (c) 2012 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 binding for the pageCapture API.
 
 var binding = require('binding').Binding.create('pageCapture');
 
 var handleUncaughtException = require('uncaught_exception_handler').handle;
 var pageCaptureNatives = requireNative('page_capture');
@@ skipping 14 matching lines @@
       handleUncaughtException(
           'Error in chrome.pageCapture.saveAsMHTML callback', e, request.stack);
     } finally {
       // Notify the browser. Now that the blob is referenced from JavaScript,
       // the browser can drop its reference to it.
       SendResponseAck(request.id);
     }
   });
 });
 
-exports.binding = binding.generate();
+exports.$set('binding', binding.generate());