[Extensions] Don't allow built-in extensions code to be overridden

extensions/renderer/resources/schema_utils.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.
 
 // Routines used to validate and normalize arguments.
 // TODO(benwells): unit test this file.
 
 var JSONSchemaValidator = require('json_schema').JSONSchemaValidator;
 
 var schemaValidator = new JSONSchemaValidator();
@@ skipping 135 matching lines @@
   var candidateSignatures = getSignatures(functionDef.parameters);
   for (var i = 0; i < candidateSignatures.length; i++) {
     for (var j = i + 1; j < candidateSignatures.length; j++) {
       if (signaturesAmbiguous(candidateSignatures[i], candidateSignatures[j]))
         return true;
     }
   }
   return false;
 };
 
-exports.isFunctionSignatureAmbiguous = isFunctionSignatureAmbiguous;
-exports.normalizeArgumentsAndValidate = normalizeArgumentsAndValidate;
-exports.schemaValidator = schemaValidator;
-exports.validate = validate;
+exports.$set('isFunctionSignatureAmbiguous', isFunctionSignatureAmbiguous);
+exports.$set('normalizeArgumentsAndValidate', normalizeArgumentsAndValidate);
+exports.$set('schemaValidator', schemaValidator);
+exports.$set('validate', validate);