Implement unset value handling

Source/core/css/CSSUnsetValue.h

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CSSUnsetValue_h
+#define CSSUnsetValue_h
+
+#include "core/css/CSSValue.h"
+#include "wtf/PassRefPtr.h"
+
+namespace blink {
+
+class CSSUnsetValue : public CSSValue {
+public:
+    static PassRefPtrWillBeRawPtr<CSSUnsetValue> create()
+    {
+        return adoptRefWillBeNoop(new CSSUnsetValue);
+    }
+
+    String customCSSText() const;
+
+    bool equals(const CSSUnsetValue&) const { return true; }
+
+    void traceAfterDispatch(Visitor* visitor) { CSSValue::traceAfterDispatch(visitor); }
+
+private:
+    CSSUnsetValue()
+        : CSSValue(UnsetClass)
+    {
+    }
+};
+
+DEFINE_CSS_VALUE_TYPE_CASTS(CSSUnsetValue, isUnsetValue());
+
+} // namespace blink
+
+#endif // CSSUnsetValue_h