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

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

Index: third_party/WebKit/Source/core/style/CounterDirectives.h
diff --git a/third_party/WebKit/Source/core/style/CounterDirectives.h b/third_party/WebKit/Source/core/style/CounterDirectives.h
index fecde4c5a0e1b4b3a51a90f56084f8f47f7db3ef..e4ecc2fc3cbd16e203493e2aa02e475110fa831b 100644
--- a/third_party/WebKit/Source/core/style/CounterDirectives.h
+++ b/third_party/WebKit/Source/core/style/CounterDirectives.h
@@ -99,9 +99,13 @@ 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