Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/wtf/PassRefPtr.h

 /*
  *  Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  *
  *  This library is free software; you can redistribute it and/or
  *  modify it under the terms of the GNU Library General Public
  *  License as published by the Free Software Foundation; either
  *  version 2 of the License, or (at your option) any later version.
  *
  *  This library is distributed in the hope that it will be useful,
  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 40 matching lines @@
         }
     }
 
     template<typename T> ALWAYS_INLINE void derefIfNotNull(T* ptr)
     {
         if (LIKELY(ptr != 0))
             ptr->deref();
     }
 
     template<typename T> class PassRefPtr {
+        WTF_DISALLOW_CONSTRUCTION_FROM_ZERO(PassRefPtr);
     public:
         PassRefPtr() : m_ptr(0) { }
+        PassRefPtr(std::nullptr_t) : m_ptr(0) { }
         PassRefPtr(T* ptr) : m_ptr(ptr) { refIfNotNull(ptr); }
         template<typename U> PassRefPtr(const RawPtr<U>& ptr, EnsurePtrConvertibleArgDecl(U, T)) : m_ptr(ptr.get()) { refIfNotNull(m_ptr); }
         explicit PassRefPtr(T& ptr) : m_ptr(&ptr) { m_ptr->ref(); }
         // It somewhat breaks the type system to allow transfer of ownership out of
         // a const PassRefPtr. However, it makes it much easier to work with PassRefPtr
         // temporaries, and we don't have a need to use real const PassRefPtrs anyway.
         PassRefPtr(const PassRefPtr& o) : m_ptr(o.leakRef()) { }
         template<typename U> PassRefPtr(const PassRefPtr<U>& o, EnsurePtrConvertibleArgDecl(U, T)) : m_ptr(o.leakRef()) { }
 
         ALWAYS_INLINE ~PassRefPtr() { derefIfNotNull(m_ptr); }
@@ skipping 104 matching lines @@
         return p.get();
     }
 
 } // namespace WTF
 
 using WTF::PassRefPtr;
 using WTF::adoptRef;
 using WTF::static_pointer_cast;
 
 #endif // WTF_PassRefPtr_h