Don't touch the prototype chain to get the private script controller.

third_party/WebKit/Source/bindings/core/v8/PrivateScriptRunner.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.
 
 "use strict";
 
 function PrivateScriptController()
 {
     this._installedClasses = {};
     this._DOMException = {};
@@ skipping 121 matching lines @@
         }
 
         var exception = new PrivateScriptException();
         exception.code = code;
         exception.message = message;
         exception.name = "PrivateScriptException";
         throw exception;
     },
 }
 
-if (typeof window.privateScriptController === 'undefined')
+if (!window.hasOwnProperty("privateScriptController"))
     window.privateScriptController = new PrivateScriptController();
 
 // This line must be the last statement of this JS file.
 // A parenthesis is needed, because the caller of this script (PrivateScriptRunner.cpp)
 // is depending on the completion value of this script.
 (privateScriptController.installedClasses);