Have CSSFontFeatureValue store the tag as an AtomicString instead of a String

Source/core/css/CSSFontFeatureValue.h

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 15 matching lines @@
 #ifndef CSSFontFeatureValue_h
 #define CSSFontFeatureValue_h
 
 #include "core/css/CSSValue.h"
 #include "wtf/text/WTFString.h"
 
 namespace WebCore {
 
 class CSSFontFeatureValue : public CSSValue {
 public:
-    static PassRefPtr<CSSFontFeatureValue> create(const String& tag, int value)
+    static PassRefPtr<CSSFontFeatureValue> create(const AtomicString& tag, int value)
     {
         return adoptRef(new CSSFontFeatureValue(tag, value));
     }
 
-    const String& tag() const { return m_tag; }
+    const AtomicString& tag() const { return m_tag; }
     int value() const { return m_value; }
     String customCSSText() const;
 
     bool equals(const CSSFontFeatureValue&) const;
 
 private:
-    CSSFontFeatureValue(const String&, int);
+    CSSFontFeatureValue(const AtomicString& tag, int value);
 
-    String m_tag;
+    AtomicString m_tag;
     const int m_value;
 };
 
 DEFINE_CSS_VALUE_TYPE_CASTS(CSSFontFeatureValue, isFontFeatureValue());
 
 } // namespace
 
 #endif