Make CPDF_Object subclass constructors intern strings

core/fxcrt/cfx_weak_ptr.h

 // Copyright 2016 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #ifndef CORE_FXCRT_CFX_WEAK_PTR_H_
 #define CORE_FXCRT_CFX_WEAK_PTR_H_
 
+#include <cstddef>
 #include <memory>
 
 #include "core/fxcrt/cfx_retain_ptr.h"
 #include "core/fxcrt/fx_system.h"
 
 template <class T, class D = std::default_delete<T>>
 class CFX_WeakPtr {
  public:
   CFX_WeakPtr() {}
   CFX_WeakPtr(const CFX_WeakPtr& that) : m_pHandle(that.m_pHandle) {}
   CFX_WeakPtr(CFX_WeakPtr&& that) { Swap(that); }
   CFX_WeakPtr(std::unique_ptr<T, D> pObj)
       : m_pHandle(new Handle(std::move(pObj))) {}
 
+  // Deliberately implicit to allow passing nullptr.
+  CFX_WeakPtr(std::nullptr_t arg) {}
+
   explicit operator bool() const { return m_pHandle && !!m_pHandle->Get(); }
   bool HasOneRef() const { return m_pHandle && m_pHandle->HasOneRef(); }
   T* operator->() { return m_pHandle->Get(); }
   const T* operator->() const { return m_pHandle->Get(); }
   CFX_WeakPtr& operator=(const CFX_WeakPtr& that) {
     m_pHandle = that.m_pHandle;
     return *this;
   }
   bool operator==(const CFX_WeakPtr& that) const {
     return m_pHandle == that.m_pHandle;
@@ skipping 37 matching lines @@
     ~Handle() {}
 
     intptr_t m_nCount;
     std::unique_ptr<T, D> m_pObj;
   };
 
   CFX_RetainPtr<Handle> m_pHandle;
 };
 
 #endif  // CORE_FXCRT_CFX_WEAK_PTR_H_