Expose CrossThreadPersistent assignment operators.

Source/heap/Handle.h

 /*
  * Copyright (C) 2014 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 197 matching lines @@
 private:
     PersistentAnchor() : PersistentNode(TraceMethodDelegate<PersistentAnchor, &PersistentAnchor::trace>::trampoline)
     {
         m_next = this;
         m_prev = this;
     }
 
     friend class ThreadState;
 };
 
+template<typename T>
+class CrossThreadPersistent;
+
 // Persistent handles are used to store pointers into the
 // managed heap. As long as the Persistent handle is alive
 // the GC will keep the object pointed to alive. Persistent
 // handles can be stored in objects and they are not scoped.
 // Persistent handles must not be used to contain pointers
 // between objects that are in the managed heap. They are only
 // meant to point to managed heap objects from variables/members
 // outside the managed heap.
 //
 // A Persistent is always a GC root from the point of view of
@@ skipping 94 matching lines @@
     Persistent& operator=(const RawPtr<U>& other)
     {
         m_raw = other;
         return *this;
     }
 
     T* get() const { return m_raw; }
 
 private:
     T* m_raw;
+
+    friend class CrossThreadPersistent<T>;
 };
 
 // Unlike Persistent, we can destruct a CrossThreadPersistent in a thread
 // different from the construction thread.
 template<typename T>
 class CrossThreadPersistent : public Persistent<T, GlobalPersistents> {
+    WTF_DISALLOW_CONSTRUCTION_FROM_ZERO(CrossThreadPersistent);
+    WTF_DISALLOW_ZERO_ASSIGNMENT(CrossThreadPersistent);
 public:
     CrossThreadPersistent(T* raw) : Persistent<T, GlobalPersistents>(raw) { }
+
+    using Persistent<T, GlobalPersistents>::operator=;
 };
 
 // FIXME: derive affinity based on the collection.
 template<typename Collection, ThreadAffinity Affinity = AnyThread>
 class PersistentHeapCollectionBase
     : public Collection
     , public PersistentBase<ThreadLocalPersistents<Affinity>, PersistentHeapCollectionBase<Collection, Affinity> > {
     // Never allocate these objects with new. Use Persistent<Collection> instead.
     DISALLOW_ALLOCATION();
 public:
@@ skipping 535 matching lines @@
 };
 
 template<typename T, typename U>
 struct NeedsTracing<HashMap<T, U> > {
     static const bool value = false;
 };
 
 } // namespace WTF
 
 #endif