Oilpan: have DocumentInit use transition types throughout.

Source/wtf/WeakPtr.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:
  * 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 67 matching lines @@
 #ifndef NDEBUG
     ThreadIdentifier m_boundThread;
 #endif
 };
 
 template<typename T>
 class WeakPtr {
     WTF_MAKE_FAST_ALLOCATED;
 public:
     WeakPtr() { }
+    WeakPtr(std::nullptr_t) { }
     WeakPtr(PassRefPtr<WeakReference<T> > ref) : m_ref(ref) { }
 
     T* get() const { return m_ref ? m_ref->get() : 0; }
     void clear() { m_ref.clear(); }
 
     T* operator->() const
     {
         ASSERT(get());
         return get();
     }
@@ skipping 45 matching lines @@
     RefPtr<WeakReference<T> > m_ref;
 };
 
 } // namespace WTF
 
 using WTF::WeakPtr;
 using WTF::WeakPtrFactory;
 using WTF::WeakReference;
 
 #endif