Oilpan: have LifetimeNotifier<T> track its observers weakly.

Source/platform/LifecycleNotifier.h

 /*
  * Copyright (C) 2008 Apple Inc. All Rights Reserved.
  * Copyright (C) 2013 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
@@ skipping 30 matching lines @@
     void addObserver(Observer*);
     void removeObserver(Observer*);
 
     // notifyContextDestroyed() should be explicitly dispatched from an
     // observed context to notify observers that contextDestroyed().
     //
     // When contextDestroyed() is called, the observer's lifecycleContext()
     // is still valid and safe to use during the notification.
     virtual void notifyContextDestroyed();
 
-    DEFINE_INLINE_VIRTUAL_TRACE() { }
+    DEFINE_INLINE_VIRTUAL_TRACE()
+    {
+#if ENABLE(OILPAN)
+        visitor->trace(m_observers);
+#endif
+    }
 
     bool isIteratingOverObservers() const { return m_iterating != IteratingNone; }
 
 protected:
     LifecycleNotifier()
         : m_iterating(IteratingNone)
         , m_didCallContextDestroyed(false)
     {
     }
 
     enum IterationType {
         IteratingNone,
         IteratingOverAll,
         IteratingOverActiveDOMObjects,
     };
 
     IterationType m_iterating;
 
 protected:
-    using ObserverSet = HashSet<Observer*>;
+    using ObserverSet = WillBeHeapHashSet<RawPtrWillBeWeakMember<Observer>>;
 
+    // FIXME: Oilpan: make LifecycleNotifier<> a GC mixin, somehow. ExecutionContext
+    // is the problematic case, as it would then be a class with two GC mixin
+    // bases, but cannot itself derive from a GC base class also.
+    GC_PLUGIN_IGNORE("467502")
     ObserverSet m_observers;
 
 #if ENABLE(ASSERT)
     T* context() { return static_cast<T*>(this); }
 #endif
 
 private:
     bool m_didCallContextDestroyed;
 };
 
@@ skipping 45 matching lines @@
 
 template<typename T, typename Observer>
 inline void LifecycleNotifier<T, Observer>::removeObserver(Observer* observer)
 {
     m_observers.remove(observer);
 }
 
 } // namespace blink
 
 #endif // LifecycleNotifier_h