Disallow adding ContextLifecycleObservers during iteration.

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 49 matching lines @@
 protected:
     LifecycleNotifier()
         : m_iterating(IteratingNone)
         , m_didCallContextDestroyed(false)
     {
     }
 
     enum IterationType {
         IteratingNone,
         IteratingOverAll,
-        IteratingOverActiveDOMObjects,
     };
 
     IterationType m_iterating;
 
 protected:
     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.
@@ skipping 56 matching lines @@
 
 template<typename T, typename Observer>
 inline void LifecycleNotifier<T, Observer>::removeObserver(Observer* observer)
 {
     m_observers.remove(observer);
 }
 
 } // namespace blink
 
 #endif // LifecycleNotifier_h