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/fxjs/xfa/cjx_subform.h b/fxjs/xfa/cjx_subform.h
index 2c2d19b..67947d8 100644
--- a/fxjs/xfa/cjx_subform.h
+++ b/fxjs/xfa/cjx_subform.h
@@ -12,7 +12,7 @@
 
 class CXFA_Delta;
 
-class CJX_Subform : public CJX_Container {
+class CJX_Subform final : public CJX_Container {
  public:
   explicit CJX_Subform(CXFA_Node* container);
   ~CJX_Subform() override;