Make down-conversion explicit from CFX_ByteString to CFX_ByteStringC.

core/fpdfapi/fpdf_parser/cpdf_number.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_number.h"
 
 CPDF_Number::CPDF_Number() : m_bInteger(TRUE), m_Integer(0) {}
 
@@ skipping 29 matching lines @@
 
 CPDF_Number* CPDF_Number::AsNumber() {
   return this;
 }
 
 const CPDF_Number* CPDF_Number::AsNumber() const {
   return this;
 }
 
 void CPDF_Number::SetString(const CFX_ByteString& str) {
-  FX_atonum(str, m_bInteger, &m_Integer);
+  FX_atonum(str.AsByteStringC(), m_bInteger, &m_Integer);
 }
 
 CFX_ByteString CPDF_Number::GetString() const {
   return m_bInteger ? CFX_ByteString::FormatInteger(m_Integer, FXFORMAT_SIGNED)
                     : CFX_ByteString::FormatFloat(m_Float);
 }