Make SelectorChecker a const operation over element.

sky/engine/core/css/ElementRuleCollector.h

Index: sky/engine/core/css/ElementRuleCollector.h
diff --git a/sky/engine/core/css/ElementRuleCollector.h b/sky/engine/core/css/ElementRuleCollector.h
index 5d811987be5aa66ddf8a9b7d1bf228cd4246e8db..4605fce62343b2092ad8427669f90ae59bfc7632 100644
--- a/sky/engine/core/css/ElementRuleCollector.h
+++ b/sky/engine/core/css/ElementRuleCollector.h
@@ -97,8 +97,6 @@ public:
     ElementRuleCollector(const ElementResolveContext&, RenderStyle* = 0);
     ~ElementRuleCollector();
 
-    void setMode(SelectorChecker::Mode mode) { m_mode = mode; }
-
     void setMatchingUARules(bool matchingUARules) { m_matchingUARules = matchingUARules; }
 
     MatchResult& matchedResult();
@@ -130,7 +128,6 @@ private:
     const ElementResolveContext& m_context;
     RefPtr<RenderStyle> m_style; // FIXME: This can be mutated during matching!
 
-    SelectorChecker::Mode m_mode;
     bool m_matchingUARules;
 
     OwnPtr<Vector<MatchedRule, 32> > m_matchedRules;