SkPDF: add `final` keyword to leaf classes

src/pdf/SkPDFFormXObject.h

Index: src/pdf/SkPDFFormXObject.h
diff --git a/src/pdf/SkPDFFormXObject.h b/src/pdf/SkPDFFormXObject.h
index 4f903f65d38cf12d4ca462de1cbbb6d15223fecf..32d9e409049334e4cea2c2d697279905d14175ec 100644
--- a/src/pdf/SkPDFFormXObject.h
+++ b/src/pdf/SkPDFFormXObject.h
@@ -30,7 +30,7 @@ class SkPDFObjNumMap;
 // The caller could keep track of the form XObjects it creates and
 // canonicalize them, but the Skia API doesn't provide enough context to
 // automatically do it (trivially).
-class SkPDFFormXObject : public SkPDFStream {
+class SkPDFFormXObject final : public SkPDFStream {
 public:
     /** Create a PDF form XObject. Entries for the dictionary entries are
      *  automatically added.