Make down-conversion explicit from CFX_ByteString to CFX_ByteStringC.

fpdfsdk/pdfwindow/PWL_Signature.cpp

 // Copyright 2014 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 "fpdfsdk/pdfwindow/PWL_Signature.h"
 
 #include "core/include/fxge/fx_ge.h"
 #include "fpdfsdk/pdfwindow/PWL_Icon.h"
@@ skipping 24 matching lines @@
     pUser2Device->Transform(rcClient.left, rcClient.top, x, y);
 
     pDevice->StretchDIBits(m_pImage, (int32_t)x, (int32_t)y,
                            (int32_t)rcClient.Width(),
                            (int32_t)rcClient.Height());
   }
 }
 
 void CPWL_Signature_Image::GetThisAppearanceStream(
     CFX_ByteTextBuf& sAppStream) {
-  sAppStream << CPWL_Image::GetImageAppStream();
+  sAppStream << CPWL_Image::GetImageAppStream().AsByteStringC();
 }
 
 void CPWL_Signature_Image::GetScale(FX_FLOAT& fHScale, FX_FLOAT& fVScale) {
   FX_FLOAT fImageW, fImageH;
 
   GetImageSize(fImageW, fImageH);
 
   CFX_FloatRect rcClient = GetClientRect();
 
   fHScale = rcClient.Width() / fImageW;
@@ skipping 113 matching lines @@
         pDevice, pUser2Device, PWL_ICONTYPE_FOXIT,
         CPWL_Utils::GetCenterSquare(GetClientRect()),
         CPWL_Color(COLORTYPE_RGB, 0.91f, 0.855f, 0.92f),
         CPWL_Color(COLORTYPE_TRANSPARENT), 255);
   }
 }
 
 void CPWL_Signature::GetThisAppearanceStream(CFX_ByteTextBuf& sAppStream) {
   CPWL_Wnd::GetThisAppearanceStream(sAppStream);
 }