Devirtualize CPDF_Object::CloneDirectObject()

This method has no overrides, and thus no reason to be virtual.

Change-Id: Ie3c565002b0b45d7d208a04d7d08d01c6b7ea0ee
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/105471
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

core/fpdfapi/parser/cpdf_object.h

diff --git a/core/fpdfapi/parser/cpdf_object.h b/core/fpdfapi/parser/cpdf_object.h
index 11ead2e..1128f69 100644
--- a/core/fpdfapi/parser/cpdf_object.h
+++ b/core/fpdfapi/parser/cpdf_object.h
@@ -57,7 +57,7 @@
 
   // Create a deep copy of the object except any reference object be
   // copied to the object it points to directly.
-  virtual RetainPtr<CPDF_Object> CloneDirectObject() const;
+  RetainPtr<CPDF_Object> CloneDirectObject() const;
 
   virtual ByteString GetString() const;
   virtual WideString GetUnicodeText() const;