Disallow direct use of AllowCrossThreadAccessWrapper

third_party/WebKit/Source/platform/CrossThreadCopier.h

 /*
  * Copyright (C) 2009, 2010 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 223 matching lines @@
     {
         return ptr;
     }
 };
 
 // |T| is |C*| or |const WeakPtr<C>&|.
 template <typename T>
 struct AllowCrossThreadAccessWrapper {
     STACK_ALLOCATED();
 public:
-    explicit AllowCrossThreadAccessWrapper(T value) : m_value(value) { }
     T value() const { return m_value; }
 private:
+    // Only constructible from AllowCrossThreadAccess().
+    explicit AllowCrossThreadAccessWrapper(T value) : m_value(value) { }
+    template <typename U>
+    friend AllowCrossThreadAccessWrapper<U*> AllowCrossThreadAccess(U*);
+    template <typename U>
+    friend AllowCrossThreadAccessWrapper<const WeakPtr<U>&> AllowCrossThreadAccess(const WeakPtr<U>&);
+
     // This raw pointer is safe since AllowCrossThreadAccessWrapper is
     // always stack-allocated. Ideally this should be Member<T> if T is
     // garbage-collected and T* otherwise, but we don't want to introduce
     // another template magic just for distinguishing Member<T> from T*.
     // From the perspective of GC, T* always works correctly.
     GC_PLUGIN_IGNORE("")
     T m_value;
 };
 
 template <typename T>
@@ skipping 11 matching lines @@
 
 template <typename T>
 AllowCrossThreadAccessWrapper<const WeakPtr<T>&> AllowCrossThreadAccess(const WeakPtr<T>& value)
 {
     return AllowCrossThreadAccessWrapper<const WeakPtr<T>&>(value);
 }
 
 } // namespace blink
 
 #endif // CrossThreadCopier_h