Return unique_ptr from CPDF_Object::Clone().

core/fpdfapi/parser/cpdf_null.cpp

 // Copyright 2016 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #include "core/fpdfapi/parser/cpdf_null.h"
+#include "third_party/base/ptr_util.h"
 
 CPDF_Null::CPDF_Null() {}
 
 CPDF_Object::Type CPDF_Null::GetType() const {
   return NULLOBJ;
 }
 
-CPDF_Object* CPDF_Null::Clone() const {
-  return new CPDF_Null;
+std::unique_ptr<CPDF_Object> CPDF_Null::Clone() const {
+  return pdfium::MakeUnique<CPDF_Null>();
 }