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_extras.h b/fxjs/xfa/cjx_extras.h
index 829aa9d..b236536 100644
--- a/fxjs/xfa/cjx_extras.h
+++ b/fxjs/xfa/cjx_extras.h
@@ -12,7 +12,7 @@
 
 class CXFA_Extras;
 
-class CJX_Extras : public CJX_Node {
+class CJX_Extras final : public CJX_Node {
  public:
   explicit CJX_Extras(CXFA_Extras* node);
   ~CJX_Extras() override;