Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/core/page/DragData.cpp

 /*
  * Copyright (C) 2007 Apple Inc.  All rights reserved.
  * Copyright (C) 2013 Google Inc.
  *
  * 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
@@ skipping 140 matching lines @@
 
     if (m_platformDragData->types().contains(mimeTypeTextHTML)) {
         String html;
         KURL baseURL;
         m_platformDragData->htmlAndBaseURL(html, baseURL);
         ASSERT(frame->document());
         if (RefPtr<DocumentFragment> fragment = createFragmentFromMarkup(*frame->document(), html, baseURL, DisallowScriptingAndPluginContent))
             return fragment.release();
     }
 
-    return 0;
+    return nullptr;
 }
 
 String DragData::droppedFileSystemId() const
 {
     DraggedIsolatedFileSystem* filesystem = DraggedIsolatedFileSystem::from(m_platformDragData);
     if (!filesystem)
         return String();
     return filesystem->filesystemId();
 }
 
 } // namespace WebCore