Oilpan: fix build++ after r192490.

Source/core/dom/DOMSettableTokenList.h

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *     notice, this list of conditions and the following disclaimer in the
@@ skipping 17 matching lines @@
 #include "core/dom/DOMTokenList.h"
 #include "core/dom/SpaceSplitString.h"
 #include "platform/heap/Handle.h"
 #include "wtf/RefCounted.h"
 #include "wtf/text/AtomicString.h"
 
 namespace blink {
 
 class ExceptionState;
 
-class DOMSettableTokenListObserver {
+class DOMSettableTokenListObserver : public WillBeGarbageCollectedMixin {
 public:
     virtual void valueChanged() = 0;
+
+    DEFINE_INLINE_VIRTUAL_TRACE() { }
 };
 
 class DOMSettableTokenList final
     : public DOMTokenList
 #if !ENABLE(OILPAN)
     , public RefCounted<DOMSettableTokenList>
 #endif
     {
     DEFINE_WRAPPERTYPEINFO();
     WTF_MAKE_FAST_ALLOCATED_WILL_BE_REMOVED(DOMSettableTokenList);
@@ skipping 14 matching lines @@
 
     virtual void add(const Vector<String>&, ExceptionState&) override;
     virtual void remove(const Vector<String>&, ExceptionState&) override;
 
     virtual const AtomicString& value() const override { return m_value; }
     virtual void setValue(const AtomicString&) override;
 
     const SpaceSplitString& tokens() const { return m_tokens; }
     void setObserver(DOMSettableTokenListObserver* observer) { m_observer = observer; };
 
+    DECLARE_VIRTUAL_TRACE();
+
 protected:
-    DOMSettableTokenList(DOMSettableTokenListObserver*);
+    explicit DOMSettableTokenList(DOMSettableTokenListObserver*);
 
 private:
     virtual void addInternal(const AtomicString&) override;
     virtual bool containsInternal(const AtomicString&) const override;
     virtual void removeInternal(const AtomicString&) override;
 
     AtomicString m_value;
     SpaceSplitString m_tokens;
-    DOMSettableTokenListObserver* m_observer;
+    RawPtrWillBeWeakMember<DOMSettableTokenListObserver> m_observer;
 };
 
 } // namespace blink
 
 #endif // DOMSettableTokenList_h