XFA: add CPDFDocumentToFPDFDocument()

fpdfsdk/include/fsdk_mgr.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_MGR_H_
 #define FPDFSDK_INCLUDE_FSDK_MGR_H_
 
 #include <map>
@@ skipping 457 matching lines @@
   CFFL_IFormFiller* m_pIFormFiller;
   IFX_SystemHandler* m_pSysHandler;
 };
 
 class CPDFSDK_Document {
  public:
   CPDFSDK_Document(CPDFXFA_Document* pDoc, CPDFDoc_Environment* pEnv);
   ~CPDFSDK_Document();
 
   CPDFSDK_InterForm* GetInterForm();
-  CPDFXFA_Document* GetDocument() { return m_pDoc; }
+  CPDFXFA_Document* GetDocument() const { return m_pDoc; }
+  CPDF_Document* GetPDFDocument() const {
+    return m_pDoc ? m_pDoc->GetPDFDoc() : nullptr;
+  }
 
   int GetPageViewCount() const { return m_pageMap.size(); }
   CPDFSDK_PageView* GetPageView(CPDFXFA_Page* pPDFXFAPage,
                                 FX_BOOL ReNew = TRUE);
   CPDFSDK_PageView* GetPageView(int nIndex);
   CPDFSDK_PageView* GetCurrentView();
   void ReMovePageView(CPDFXFA_Page* pPDFPage);
   void UpdateAllViews(CPDFSDK_PageView* pSender, CPDFSDK_Annot* pAnnot);
 
   CPDFSDK_Annot* GetFocusAnnot();
@@ skipping 180 matching lines @@
     }
 
     if (nStartPos < m)
       QuickSort(nStartPos, m, bAscend, pCompare);
     if (nStopPos > m)
       QuickSort(m, nStopPos, bAscend, pCompare);
   }
 };
 
 #endif  // FPDFSDK_INCLUDE_FSDK_MGR_H_