Support WeakMember<const T>.

third_party/WebKit/Source/platform/heap/Visitor.h

 /*
  * Copyright (C) 2013 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 190 matching lines @@
     // callback for each cell that needs to be zeroed, so if you have a lot of
     // weak cells in your object you should still consider using
     // registerWeakMembers above.
     //
     // In contrast to registerWeakMembers, the weak cell callbacks are
     // run on the thread performing garbage collection. Therefore, all
     // threads are stopped during weak cell callbacks.
     template<typename T>
     void registerWeakCell(T** cell)
     {
-        Derived::fromHelper(this)->registerWeakCellWithCallback(reinterpret_cast<void**>(cell), &handleWeakCell<T>);
+        Derived::fromHelper(this)->registerWeakCellWithCallback(reinterpret_cast<void**>(const_cast<typename std::remove_const<T>::type**>(cell)), &handleWeakCell<T>);
     }
 
     template<typename T, void (T::*method)(Visitor*)>
     void registerWeakMembers(const T* obj)
     {
         registerWeakMembers(obj, &TraceMethodDelegate<T, method>::trampoline);
     }
     void registerWeakMembers(const void* object, WeakCallback callback)
     {
         Derived::fromHelper(this)->registerWeakMembers(object, object, callback);
@@ skipping 101 matching lines @@
 private:
     static Visitor* fromHelper(VisitorHelper<Visitor>* helper) { return static_cast<Visitor*>(helper); }
 
     ThreadState* m_state;
     const MarkingMode m_markingMode;
 };
 
 } // namespace blink
 
 #endif // Visitor_h