Make CounterDirectiveMap a class and implement a Clone() method.

third_party/WebKit/Source/core/style/CounterDirectives.h

 /*
  * Copyright (C) 2000 Lars Knoll (knoll@kde.org)
  *           (C) 2000 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
@@ skipping 81 matching lines @@
   bool is_increment_set_;
   int reset_value_;
   int increment_value_;
 };
 
 bool operator==(const CounterDirectives&, const CounterDirectives&);
 inline bool operator!=(const CounterDirectives& a, const CounterDirectives& b) {
   return !(a == b);
 }
 
-typedef HashMap<AtomicString, CounterDirectives> CounterDirectiveMap;
-
-std::unique_ptr<CounterDirectiveMap> Clone(const CounterDirectiveMap&);
+// Not to be deleted through a pointer to HashMap.
+class CounterDirectiveMap : public HashMap<AtomicString, CounterDirectives> {
+ public:
+  std::unique_ptr<CounterDirectiveMap> Clone() const {
+    return WTF::WrapUnique(new CounterDirectiveMap(*this));
+  }
+};
 
 }  // namespace blink
 
 #endif  // CounterDirectives_h