Clean up virtual functions in css/

Source/core/css/CSSUnknownRule.h

Index: Source/core/css/CSSUnknownRule.h
diff --git a/Source/core/css/CSSUnknownRule.h b/Source/core/css/CSSUnknownRule.h
index e5f704b5ad87fe931e9abb8accb3b9d9c1897745..b6bb13dae8038c2352f9bd3a5026f65ac76a2fb0 100644
--- a/Source/core/css/CSSUnknownRule.h
+++ b/Source/core/css/CSSUnknownRule.h
@@ -32,10 +32,12 @@ public:
     CSSUnknownRule() : CSSRule(0) { }
     virtual ~CSSUnknownRule() { }
 
-    virtual CSSRule::Type type() const override { return UNKNOWN_RULE; }
     virtual String cssText() const override { return String(); }
     virtual void reattach(StyleRuleBase*) override { }
     virtual void trace(Visitor* visitor) override { CSSRule::trace(visitor); }
+
+private:
+    virtual CSSRule::Type type() const override { return UNKNOWN_RULE; }
 };
 
 } // namespace blink