Oilpan: fix build after r180932.

Source/core/css/TreeBoundaryCrossingRules.h

Index: Source/core/css/TreeBoundaryCrossingRules.h
diff --git a/Source/core/css/TreeBoundaryCrossingRules.h b/Source/core/css/TreeBoundaryCrossingRules.h
index 08b59aa39b14af407ac84a14ca1c5a3cc08a9ef5..8f907c31e229394f922d75641b26a4effdf4d815 100644
--- a/Source/core/css/TreeBoundaryCrossingRules.h
+++ b/Source/core/css/TreeBoundaryCrossingRules.h
@@ -50,15 +50,18 @@ public:
 
 private:
     size_t size() const { return m_scopingNodes.size(); }
-    class RuleSubSet {
+    class RuleSubSet FINAL : public NoBaseWillBeGarbageCollected<RuleSubSet> {
     public:
         static PassOwnPtrWillBeRawPtr<RuleSubSet> create(CSSStyleSheet* sheet, unsigned index, PassOwnPtrWillBeRawPtr<RuleSet> rules)
         {
             return adoptPtrWillBeNoop(new RuleSubSet(sheet, index, rules));
         }
+
         CSSStyleSheet* parentStyleSheet;
         unsigned parentIndex;
-        OwnPtr<RuleSet> ruleSet;
+        OwnPtrWillBeMember<RuleSet> ruleSet;
+
+        void trace(Visitor*);
 
     private:
         RuleSubSet(CSSStyleSheet* sheet, unsigned index, PassOwnPtrWillBeRawPtr<RuleSet> rules)