[ios] Use square brackets syntax for nulling out toJSON property.

ios/web/web_state/js/resources/common.js

 // Copyright 2013 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.
 
 // This file provides common methods that can be shared by other JavaScripts.
 
 goog.provide('__crWeb.common');
 
 goog.require('__crWeb.base');
 
@@ skipping 19 matching lines @@
    * JSON safe object to protect against custom implementation of Object.toJSON
    * in host pages.
    * @constructor
    */
   __gCrWeb.common.JSONSafeObject = function JSONSafeObject() {
   };
 
   /**
    * Protect against custom implementation of Object.toJSON in host pages.
    */
-  __gCrWeb.common.JSONSafeObject.prototype.toJSON = null;
+  __gCrWeb.common.JSONSafeObject.prototype['toJSON'] = null;
 
   /**
    * Retain the original JSON.stringify method where possible to reduce the
    * impact of sites overriding it
    */
   __gCrWeb.common.JSONStringify = JSON.stringify;
 
   /**
    * Prefix used in references to form elements that have no 'id' or 'name'
    */
@@ skipping 632 matching lines @@
     var plugins = findPluginNodesWithoutFallback_();
     if (plugins.length > 0) {
       // Store the list of plugins in a known place for the replacement script
       // to use, then trigger it.
       __gCrWeb['placeholderTargetPlugins'] = plugins;
       return true;
     }
     return false;
   };
 }());  // End of anonymous object