Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/wtf/text/StringConcatenate.h

 /*
  * Copyright (C) 2010 Apple 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 387 matching lines @@
 template<typename StringType1, typename StringType2>
 PassRefPtr<StringImpl> makeString(StringType1 string1, StringType2 string2)
 {
     StringTypeAdapter<StringType1> adapter1(string1);
     StringTypeAdapter<StringType2> adapter2(string2);
 
     bool overflow = false;
     unsigned length = adapter1.length();
     sumWithOverflow(length, adapter2.length(), overflow);
     if (overflow)
-        return 0;
+        return nullptr;
 
     if (adapter1.is8Bit() && adapter2.is8Bit()) {
         LChar* buffer;
         RefPtr<StringImpl> resultImpl = StringImpl::createUninitialized(length, buffer);
         if (!resultImpl)
-            return 0;
+            return nullptr;
 
         LChar* result = buffer;
         adapter1.writeTo(result);
         result += adapter1.length();
         adapter2.writeTo(result);
 
         return resultImpl.release();
     }
 
     UChar* buffer;
     RefPtr<StringImpl> resultImpl = StringImpl::createUninitialized(length, buffer);
     if (!resultImpl)
-        return 0;
+        return nullptr;
 
     UChar* result = buffer;
     adapter1.writeTo(result);
     result += adapter1.length();
     adapter2.writeTo(result);
 
     return resultImpl.release();
 }
 
 } // namespace WTF
 
 #include "wtf/text/StringOperators.h"
 #endif