Make CFX_WideString(const CFX_WideString&) explicit.

xfa/fxbarcode/cbc_code39.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
 /*
  * Copyright 2011 ZXing authors
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ skipping 91 matching lines @@
                                   int32_t& e) {
   CFX_WideString str;
   return str;
 }
 
 CFX_WideString CBC_Code39::Decode(CFX_DIBitmap* pBitmap, int32_t& e) {
   CBC_BufferedImageLuminanceSource source(pBitmap);
   CBC_GlobalHistogramBinarizer binarizer(&source);
   CBC_BinaryBitmap bitmap(&binarizer);
   CFX_ByteString str = m_pBCReader->Decode(&bitmap, 0, e);
-  BC_EXCEPTION_CHECK_ReturnValue(e, FX_WSTRC(L""));
+  BC_EXCEPTION_CHECK_ReturnValue(e, CFX_WideString());
   return CFX_WideString::FromUTF8(str.AsStringC());
 }
 
 FX_BOOL CBC_Code39::SetTextLocation(BC_TEXT_LOC location) {
   if (m_pBCWriter)
     return ((CBC_OnedCode39Writer*)m_pBCWriter)->SetTextLocation(location);
   return FALSE;
 }
 
 FX_BOOL CBC_Code39::SetWideNarrowRatio(int32_t ratio) {
   if (m_pBCWriter)
     return ((CBC_OnedCode39Writer*)m_pBCWriter)->SetWideNarrowRatio(ratio);
   return FALSE;
 }