Oilpan: Move LifecycleNotifier's hierarchy to Oilpan's heap

Source/core/dom/DocumentLifecycleNotifier.h

Index: Source/core/dom/DocumentLifecycleNotifier.h
diff --git a/Source/core/dom/DocumentLifecycleNotifier.h b/Source/core/dom/DocumentLifecycleNotifier.h
index 229002a212b63bd1db64590e5c99ab4877f5d95b..ca933d0f02f00600243c397914c4d844580e704e 100644
--- a/Source/core/dom/DocumentLifecycleNotifier.h
+++ b/Source/core/dom/DocumentLifecycleNotifier.h
@@ -46,11 +46,13 @@ public:
     virtual void addObserver(Observer*) override final;
     virtual void removeObserver(Observer*) override final;
 
+    virtual void trace(Visitor*) override;
+
 protected:
     explicit DocumentLifecycleNotifier(Document*);
 
 private:
-    using DocumentObserverSet = HashSet<DocumentLifecycleObserver*>;
+    using DocumentObserverSet = WillBeHeapHashSet<RawPtrWillBeWeakMember<DocumentLifecycleObserver>>;
     DocumentObserverSet m_documentObservers;
 };