SkPDF: add `final` keyword to leaf classes

src/pdf/SkPDFTypes.h

Index: src/pdf/SkPDFTypes.h
diff --git a/src/pdf/SkPDFTypes.h b/src/pdf/SkPDFTypes.h
index 0f029980b17faac001b6ad85a277310d3018bcf4..99320b1efc77708347d3f71e96a314a466872813 100644
--- a/src/pdf/SkPDFTypes.h
+++ b/src/pdf/SkPDFTypes.h
@@ -168,7 +168,7 @@ static_assert(sizeof(SkString) == sizeof(void*), "SkString_size");
 /** This class is a SkPDFUnion with SkPDFObject virtuals attached.
     The only use case of this is when a non-compound PDF object is
     referenced indirectly. */
-class SkPDFAtom : public SkPDFObject {
+class SkPDFAtom final : public SkPDFObject {
 public:
     void emitObject(SkWStream* stream,
                     const SkPDFObjNumMap& objNumMap,
@@ -188,7 +188,7 @@ private:
 
     An array object in a PDF.
 */
-class SkPDFArray : public SkPDFObject {
+class SkPDFArray final : public SkPDFObject {
 public:
     static const int kMaxLen = 8191;
 
@@ -314,7 +314,7 @@ private:
     descriptor). That is: no memory savings can be made by holding on
     to a compressed version instead.
  */
-class SkPDFSharedStream : public SkPDFObject {
+class SkPDFSharedStream final : public SkPDFObject {
 public:
     // Takes ownership of asset.
     SkPDFSharedStream(SkStreamAsset* data) : fAsset(data), fDict(new SkPDFDict) { SkASSERT(data); }