Introduce "underlying types" to abstract XFA differences.

fpdfsdk/include/fsdk_baseannot.h

Index: fpdfsdk/include/fsdk_baseannot.h
diff --git a/fpdfsdk/include/fsdk_baseannot.h b/fpdfsdk/include/fsdk_baseannot.h
index e9a33129b0e932e3cb5d73efbb4ff5af754d91b6..441036a657b9e406bd3d0f25c0681b3b1419079e 100644
--- a/fpdfsdk/include/fsdk_baseannot.h
+++ b/fpdfsdk/include/fsdk_baseannot.h
@@ -15,7 +15,8 @@
 
 #include "core/include/fpdfdoc/fpdf_doc.h"
 #include "core/include/fxcrt/fx_basic.h"
-#include "fx_systemhandler.h"
+#include "fpdfsdk/include/fsdk_define.h"
+#include "fpdfsdk/include/fx_systemhandler.h"
 
 class CPDFSDK_PageView;
 class CPDF_Annot;
@@ -91,6 +92,7 @@ class CPDFSDK_Annot {
                             CPDF_Matrix* pUser2Device,
                             CPDF_RenderOptions* pOptions) {}
 
+  UnderlyingPageType* GetUnderlyingPage();
   CPDF_Page* GetPDFPage();
   CPDFXFA_Page* GetPDFXFAPage();