The final game: mark everything final.

Then revert the ones that break compilation.
Fix one IWYU noticed during presubmit.

Change-Id: I881a8a72818e55dbc4816247e35ff5e3015194e7
Reviewed-on: https://pdfium-review.googlesource.com/41470
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/xfa/fxfa/parser/cxfa_pdf.h b/xfa/fxfa/parser/cxfa_pdf.h
index d0925c8..00e4d38 100644
--- a/xfa/fxfa/parser/cxfa_pdf.h
+++ b/xfa/fxfa/parser/cxfa_pdf.h
@@ -9,7 +9,7 @@
 
 #include "xfa/fxfa/parser/cxfa_node.h"
 
-class CXFA_Pdf : public CXFA_Node {
+class CXFA_Pdf final : public CXFA_Node {
  public:
   CXFA_Pdf(CXFA_Document* doc, XFA_PacketType packet);
   ~CXFA_Pdf() override;