Blink-in-JS: Support partial interfaces in private scripts

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";
 
 // This list must be in sync with the enum in ExceptionCode.h.  The order matters.
 var domExceptions = [
     "IndexSizeError",
     "HierarchyRequestError",
@@ skipping 64 matching lines @@
     this.message = message;
     this.name = "DOMExceptionInPrivateScript";
 }
 
 function privateScriptClass()
 {
 }
 
 function installClass(className, implementation)
 {
-    installedClasses[className] = new privateScriptClass();
+    if (installedClasses[className] === undefined)
+        installedClasses[className] = new privateScriptClass();
     implementation(window, installedClasses[className]);
 }
 
 init();
 
 // 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.
 (installedClasses);