Add CPDF_Document::Extension::GetPageCount()

Another virtual API at the CPDF layer, to avoid a compile time
ifdef XFA.

Change-Id: Ia95c4d3b3d3b773aaf45c49ebcadff6b16ca18c6
Reviewed-on: https://pdfium-review.googlesource.com/31910
Commit-Queue: dsinclair <dsinclair@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
diff --git a/fpdfsdk/fpdf_view.cpp b/fpdfsdk/fpdf_view.cpp
index 1eb014a..ea12bf9 100644
--- a/fpdfsdk/fpdf_view.cpp
+++ b/fpdfsdk/fpdf_view.cpp
@@ -329,13 +329,8 @@
   if (!pDoc)
     return 0;
 
-#ifdef PDF_ENABLE_XFA
-  auto* pContext = static_cast<CPDFXFA_Context*>(pDoc->GetExtension());
-  if (pContext)
-    return pContext->GetPageCount();
-#endif  // PDF_ENABLE_XFA
-
-  return pDoc->GetPageCount();
+  auto* pExtension = pDoc->GetExtension();
+  return pExtension ? pExtension->GetPageCount() : pDoc->GetPageCount();
 }
 
 FPDF_EXPORT FPDF_PAGE FPDF_CALLCONV FPDF_LoadPage(FPDF_DOCUMENT document,