Make down-conversion explicit from CFX_ByteString to CFX_ByteStringC.

core/fpdfapi/fpdf_parser/include/cpdf_object.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_FPDFAPI_FPDF_PARSER_INCLUDE_CPDF_OBJECT_H_
 #define CORE_FPDFAPI_FPDF_PARSER_INCLUDE_CPDF_OBJECT_H_
 
 #include "core/fxcrt/include/fx_string.h"
@@ skipping 28 matching lines @@
   uint32_t GetObjNum() const { return m_ObjNum; }
   uint32_t GetGenNum() const { return m_GenNum; }
 
   virtual CPDF_Object* Clone(FX_BOOL bDirect = FALSE) const = 0;
   virtual CPDF_Object* GetDirect() const;
 
   FX_BOOL IsModified() const { return FALSE; }
   void Release();
 
   virtual CFX_ByteString GetString() const;
+
+  // Note: |this| must outlive the use of |GetConstString|'s result.
   virtual CFX_ByteStringC GetConstString() const;
   virtual CFX_WideString GetUnicodeText() const;
   virtual FX_FLOAT GetNumber() const;
   virtual int GetInteger() const;
   virtual CPDF_Dictionary* GetDict() const;
   virtual CPDF_Array* GetArray() const;
 
   virtual void SetString(const CFX_ByteString& str);
 
   virtual bool IsArray() const;
@@ skipping 94 matching lines @@
 
 inline CPDF_Stream* ToStream(CPDF_Object* obj) {
   return obj ? obj->AsStream() : nullptr;
 }
 
 inline const CPDF_Stream* ToStream(const CPDF_Object* obj) {
   return obj ? obj->AsStream() : nullptr;
 }
 
 #endif  // CORE_FPDFAPI_FPDF_PARSER_INCLUDE_CPDF_OBJECT_H_