Oilpan: fix build after r180932.

Source/core/css/TreeBoundaryCrossingRules.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 32 matching lines @@
     void addTreeBoundaryCrossingRules(const RuleSet&, CSSStyleSheet*, unsigned sheetIndex, ContainerNode&);
 
     void reset(const ContainerNode* scopingNode);
     void collectFeaturesTo(RuleFeatureSet&);
     void collectTreeBoundaryCrossingRules(Element*, ElementRuleCollector&, bool includeEmptyRules);
 
     void trace(Visitor*);
 
 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)
             : parentStyleSheet(sheet)
             , parentIndex(index)
             , ruleSet(rules)
         {
         }
     };
     typedef WillBeHeapVector<OwnPtrWillBeMember<RuleSubSet> > CSSStyleSheetRuleSubSet;
     void collectFeaturesFromRuleSubSet(CSSStyleSheetRuleSubSet*, RuleFeatureSet&);
 
     DocumentOrderedList m_scopingNodes;
     typedef WillBeHeapHashMap<RawPtrWillBeMember<const ContainerNode>, OwnPtrWillBeMember<CSSStyleSheetRuleSubSet> > TreeBoundaryCrossingRuleSetMap;
     TreeBoundaryCrossingRuleSetMap m_treeBoundaryCrossingRuleSetMap;
 };
 
 } // namespace blink
 
 #endif // TreeBoundaryCrossingRules_h