Make down-conversion explicit from CFX_ByteString to CFX_ByteStringC.

core/fpdfapi/fpdf_parser/cpdf_string.cpp

 // 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
 
 #include "core/fpdfapi/fpdf_parser/include/cpdf_string.h"
 
 #include "core/fpdfapi/fpdf_parser/include/fpdf_parser_decode.h"
 
@@ skipping 14 matching lines @@
 
 CPDF_Object* CPDF_String::Clone(FX_BOOL bDirect) const {
   return new CPDF_String(m_String, m_bHex);
 }
 
 CFX_ByteString CPDF_String::GetString() const {
   return m_String;
 }
 
 CFX_ByteStringC CPDF_String::GetConstString() const {
-  return CFX_ByteStringC(m_String);
+  return m_String.AsByteStringC();
 }
 
 void CPDF_String::SetString(const CFX_ByteString& str) {
   m_String = str;
 }
 
 bool CPDF_String::IsString() const {
   return true;
 }
 
 CPDF_String* CPDF_String::AsString() {
   return this;
 }
 
 const CPDF_String* CPDF_String::AsString() const {
   return this;
 }
 
 CFX_WideString CPDF_String::GetUnicodeText() const {
   return PDF_DecodeText(m_String);
 }