Introduce "underlying types" to abstract XFA differences.

fpdfsdk/include/fsdk_define.h

Index: fpdfsdk/include/fsdk_define.h
diff --git a/fpdfsdk/include/fsdk_define.h b/fpdfsdk/include/fsdk_define.h
index 389e8e5098cea2893783ca15f5307e45da469e43..89c8f6ca6aaa230bfd1096ec4230c85e2ba238a6 100644
--- a/fpdfsdk/include/fsdk_define.h
+++ b/fpdfsdk/include/fsdk_define.h
@@ -22,6 +22,8 @@
 #include "core/include/fpdfdoc/fpdf_vt.h"
 #include "core/include/fxge/fx_ge.h"
 #include "core/include/fxge/fx_ge_win32.h"
+#include "fpdfsdk/include/fpdfxfa/fpdfxfa_doc.h"
+#include "fpdfsdk/include/fpdfxfa/fpdfxfa_page.h"
 #include "public/fpdfview.h"
 
 #ifdef _WIN32
@@ -87,6 +89,18 @@ class CFPDF_FileStream : public IFX_FileStream {
   FX_FILESIZE m_nCurPos;
 };
 
+// Object types for public FPDF_ types; these correspond to next layer down
+// from fpdfsdk. For master, these are CPDF_ types, but for XFA, these are
+// CPDFXFA_ types.
+using UnderlyingDocumentType = CPDFXFA_Document;
+using UnderlyingPageType = CPDFXFA_Page;
+
+// Conversions to/from underlying types.
+UnderlyingDocumentType* UnderlyingFromFPDFDocument(FPDF_DOCUMENT doc);
+FPDF_DOCUMENT FPDFDocumentFromUnderlying(UnderlyingDocumentType* doc);
+
+UnderlyingPageType* UnderlyingFromFPDFPage(FPDF_PAGE page);
+
 // Conversions to/from FPDF_ types.
 CPDF_Document* CPDFDocumentFromFPDFDocument(FPDF_DOCUMENT doc);
 FPDF_DOCUMENT FPDFDocumentFromCPDFDocument(CPDF_Document* doc);