XFA: add CPDFDocumentToFPDFDocument()

fpdfsdk/include/fsdk_define.h

 // 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
 
 #ifndef FPDFSDK_INCLUDE_FSDK_DEFINE_H_
 #define FPDFSDK_INCLUDE_FSDK_DEFINE_H_
 
 #include "../../core/include/fpdfapi/fpdf_module.h"
@@ skipping 69 matching lines @@
   virtual FX_BOOL WriteBlock(const void* buffer,
                              FX_FILESIZE offset,
                              size_t size);
   virtual FX_BOOL Flush();
 
  protected:
   FPDF_FILEHANDLER* m_pFS;
   FX_FILESIZE m_nCurPos;
 };
 
-// Conversions from FPDF_ types.
+// Conversions to/from FPDF_ types.
 CPDF_Document* CPDFDocumentFromFPDFDocument(FPDF_DOCUMENT doc);
+FPDF_DOCUMENT FPDFDocumentFromCPDFDocument(CPDF_Document* doc);
+
 CPDF_Page* CPDFPageFromFPDFPage(FPDF_PAGE page);
 
 void DropContext(void* data);
 void FSDK_SetSandBoxPolicy(FPDF_DWORD policy, FPDF_BOOL enable);
 FPDF_BOOL FSDK_IsSandBoxPolicyEnabled(FPDF_DWORD policy);
 void FPDF_RenderPage_Retail(CRenderContext* pContext,
                             FPDF_PAGE page,
                             int start_x,
                             int start_y,
                             int size_x,
                             int size_y,
                             int rotate,
                             int flags,
                             FX_BOOL bNeedToRestore,
                             IFSDK_PAUSE_Adapter* pause);
 
 void CheckUnSupportError(CPDF_Document* pDoc, FX_DWORD err_code);
 void CheckUnSupportAnnot(CPDF_Document* pDoc, CPDF_Annot* pPDFAnnot);
 void ProcessParseError(FX_DWORD err_code);
 
 #endif  // FPDFSDK_INCLUDE_FSDK_DEFINE_H_