Oilpan: Prepare to move IdTargetObserver and IdTargetObserverRegistry to Oilpan heap.

Source/core/dom/IdTargetObserver.h

Index: Source/core/dom/IdTargetObserver.h
diff --git a/Source/core/dom/IdTargetObserver.h b/Source/core/dom/IdTargetObserver.h
index 1fc4cd6f57520d79965e372712e91ede0dfd34f8..71ee604cde4f3f0535315d83d653bb01464370d9 100644
--- a/Source/core/dom/IdTargetObserver.h
+++ b/Source/core/dom/IdTargetObserver.h
@@ -26,15 +26,17 @@
 #ifndef IdTargetObserver_h
 #define IdTargetObserver_h
 
+#include "platform/heap/Handle.h"
 #include "wtf/text/AtomicString.h"
 
 namespace WebCore {
 
 class IdTargetObserverRegistry;
 
-class IdTargetObserver {
+class IdTargetObserver : public NoBaseWillBeGarbageCollectedFinalized<IdTargetObserver> {
 public:
     virtual ~IdTargetObserver();
+    virtual void trace(Visitor*);
     virtual void idTargetChanged() = 0;
     virtual void unregister();
 
@@ -42,7 +44,9 @@ protected:
     IdTargetObserver(IdTargetObserverRegistry&, const AtomicString& id);
 
 private:
-    IdTargetObserverRegistry& m_registry;
+    IdTargetObserverRegistry& registry() { return *m_registry; }
+
+    RawPtrWillBeMember<IdTargetObserverRegistry> m_registry;
     AtomicString m_id;
 };