Introduce "underlying types" to abstract XFA differences.

fpdfsdk/src/fpdfview.cpp

Index: fpdfsdk/src/fpdfview.cpp
diff --git a/fpdfsdk/src/fpdfview.cpp b/fpdfsdk/src/fpdfview.cpp
index 8da56dc3b6a00eefc48a1c7994b96e9e06cb8218..7dc17262c63d02b2b4677246a9d154e9fe3710a4 100644
--- a/fpdfsdk/src/fpdfview.cpp
+++ b/fpdfsdk/src/fpdfview.cpp
@@ -23,16 +23,30 @@
 #include "third_party/base/nonstd_unique_ptr.h"
 #include "third_party/base/numerics/safe_conversions_impl.h"
 
+UnderlyingDocumentType* UnderlyingFromFPDFDocument(FPDF_DOCUMENT doc) {
+  return static_cast<UnderlyingDocumentType*>(doc);
+}
+
+FPDF_DOCUMENT FPDFDocumentFromUnderlying(UnderlyingDocumentType* doc) {
+  return static_cast<FPDF_DOCUMENT>(doc);
+}
+
+UnderlyingPageType* UnderlyingFromFPDFPage(FPDF_PAGE page) {
+  return static_cast<UnderlyingPageType*>(page);
+}
+
 CPDF_Document* CPDFDocumentFromFPDFDocument(FPDF_DOCUMENT doc) {
-  return doc ? static_cast<CPDFXFA_Document*>(doc)->GetPDFDoc() : nullptr;
+  return doc ? UnderlyingFromFPDFDocument(doc)->GetPDFDoc() : nullptr;
 }
 
 FPDF_DOCUMENT FPDFDocumentFromCPDFDocument(CPDF_Document* doc) {
-  return doc ? new CPDFXFA_Document(doc, CPDFXFA_App::GetInstance()) : nullptr;
+  return doc ? FPDFDocumentFromUnderlying(
+                   new CPDFXFA_Document(doc, CPDFXFA_App::GetInstance()))
+             : nullptr;
 }
 
 CPDF_Page* CPDFPageFromFPDFPage(FPDF_PAGE page) {
-  return page ? static_cast<CPDFXFA_Page*>(page)->GetPDFPage() : nullptr;
+  return page ? UnderlyingFromFPDFPage(page)->GetPDFPage() : nullptr;
 }
 
 CFPDF_FileStream::CFPDF_FileStream(FPDF_FILEHANDLER* pFS) {