Make CPDF_Object subclass constructors intern strings

core/fxcrt/cfx_weak_ptr.h

Index: core/fxcrt/cfx_weak_ptr.h
diff --git a/core/fxcrt/cfx_weak_ptr.h b/core/fxcrt/cfx_weak_ptr.h
index 9ac1475884d0f0f9b7194f118d4845e38fd25dbd..f679696992b93bd4fe0f72fb8821c50e4d3f071c 100644
--- a/core/fxcrt/cfx_weak_ptr.h
+++ b/core/fxcrt/cfx_weak_ptr.h
@@ -7,6 +7,7 @@
 #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"
@@ -21,6 +22,9 @@ class CFX_WeakPtr {
   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(); }