Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/wtf/RawPtr.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 26 matching lines @@
 // interface (get, clear) of other pointer types such as RefPtr,
 // Persistent and Member. This is used for the Blink garbage
 // collection work in order to be able to write shared code that will
 // use reference counting or garbage collection based on a
 // compile-time flag.
 
 namespace WTF {
 
 template<typename T>
 class RawPtr {
+    WTF_DISALLOW_CONSTRUCTION_FROM_ZERO(RawPtr);
+    WTF_DISALLOW_ZERO_ASSIGNMENT(RawPtr);
 public:
     RawPtr() : m_ptr(0) { }
+    RawPtr(std::nullptr_t) : m_ptr(0) { }
     RawPtr(T* ptr) : m_ptr(ptr) { }
     RawPtr(const RawPtr& other)
         : m_ptr(other.get())
     {
     }
     template<typename U>
     RawPtr(const RawPtr<U>& other)
         : m_ptr(other.get())
     {
     }
@@ skipping 21 matching lines @@
         return *this;
     }
 
     template<typename U>
     RawPtr& operator=(RawPtr<U> ptr)
     {
         m_ptr = ptr.get();
         return *this;
     }
 
+    RawPtr& operator=(std::nullptr_t)
+    {
+        m_ptr = 0;
+        return *this;
+    }
+
     operator T*() const { return m_ptr; }
     T& operator*() const { return *m_ptr; }
     T* operator->() const { return m_ptr; }
 
     void swap(RawPtr& o)
     {
         std::swap(m_ptr, o.m_ptr);
     }
 
 
 private:
     T* m_ptr;
 };
 
 template<typename T, typename U> inline RawPtr<T> static_pointer_cast(const RawPtr<U>& p)
 {
     return RawPtr<T>(static_cast<T*>(p.get()));
 }
 
 } // namespace WTF
 
 using WTF::RawPtr;
 
 #endif