Reland of Clear LifecycleObserver::m_context when LifecycleObserver::contextDestroyed gets called

third_party/WebKit/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 43 matching lines @@
     }
 
     bool isIteratingOverObservers() const { return m_iterationState != NotIterating; }
 
 protected:
     LifecycleNotifier()
         : m_iterationState(NotIterating)
     {
     }
 
-#if DCHECK_IS_ON()
     T* context() { return static_cast<T*>(this); }
-#endif
 
     using ObserverSet = HeapHashSet<WeakMember<Observer>>;
 
     void removePending(ObserverSet&);
 
     enum IterationState {
         AllowingNone        = 0,
         AllowingAddition    = 1,
         AllowingRemoval     = 2,
         NotIterating        = AllowingAddition | AllowingRemoval,
@@ skipping 14 matching lines @@
 }
 
 template<typename T, typename Observer>
 inline void LifecycleNotifier<T, Observer>::notifyContextDestroyed()
 {
     // Observer unregistration is allowed, but effectively a no-op.
     AutoReset<IterationState> scope(&m_iterationState, AllowingRemoval);
     ObserverSet observers;
     m_observers.swap(observers);
     for (Observer* observer : observers) {
-        ASSERT(observer->lifecycleContext() == context());
+        DCHECK(observer->lifecycleContext() == context());
         observer->contextDestroyed();
+        observer->clearContext();
     }
 }
 
 template<typename T, typename Observer>
 inline void LifecycleNotifier<T, Observer>::addObserver(Observer* observer)
 {
     RELEASE_ASSERT(m_iterationState & AllowingAddition);
     m_observers.add(observer);
 }
 
@@ skipping 18 matching lines @@
         // the table is likely to become garbage soon.
         ThreadState::NoAllocationScope scope(ThreadState::current());
         observers.removeAll(m_observers);
     }
     m_observers.swap(observers);
 }
 
 } // namespace blink
 
 #endif // LifecycleNotifier_h