Introduce "underlying types" to abstract XFA differences.

fpdfsdk/src/fsdk_baseannot.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 "core/include/fxcrt/fx_ext.h"
 #include "fpdfsdk/include/fpdfxfa/fpdfxfa_doc.h"
 #include "fpdfsdk/include/fsdk_baseannot.h"
 #include "fpdfsdk/include/fsdk_define.h"
@@ skipping 963 matching lines @@
 void CPDFSDK_BAAnnot::Annot_OnDraw(CFX_RenderDevice* pDevice,
                                    CPDF_Matrix* pUser2Device,
                                    CPDF_RenderOptions* pOptions) {
   m_pAnnot->GetAPForm(m_pPageView->GetPDFPage(), CPDF_Annot::Normal);
   m_pAnnot->DrawAppearance(m_pPageView->GetPDFPage(), pDevice, pUser2Device,
                            CPDF_Annot::Normal, NULL);
 
   return;
 }
 
+UnderlyingPageType* CPDFSDK_Annot::GetUnderlyingPage() {
+  return GetPDFXFAPage();
+}
+
 CPDF_Page* CPDFSDK_Annot::GetPDFPage() {
   if (m_pPageView)
     return m_pPageView->GetPDFPage();
   return NULL;
 }
 
 CPDFXFA_Page* CPDFSDK_Annot::GetPDFXFAPage() {
   if (m_pPageView)
     return m_pPageView->GetPDFXFAPage();
   return NULL;
 }