LifecycleObserver::contextDestroyed should be explicitly dispatched promptly when a context is dest…

Source/platform/LifecycleNotifier.h

Index: Source/platform/LifecycleNotifier.h
diff --git a/Source/platform/LifecycleNotifier.h b/Source/platform/LifecycleNotifier.h
index 818cd931b998d9a6583f08f1bc2a21d13b8a1c87..4a638f5eb1c7383d207d7e97b9f5e0e6e241d8b6 100644
--- a/Source/platform/LifecycleNotifier.h
+++ b/Source/platform/LifecycleNotifier.h
@@ -45,9 +45,13 @@ public:
         return adoptPtr(new LifecycleNotifier(context));
     }
 
+    virtual ~LifecycleNotifier()
+    {
+        ASSERT(!m_observers.size());
+        ASSERT(!m_context);
+    }
 
-    virtual ~LifecycleNotifier();
-
+    void contextDestroyed();
 
     // FIXME: this won't need to be virtual anymore.
     virtual void addObserver(Observer*);
@@ -84,7 +88,7 @@ private:
 };
 
 template<typename T>
-inline LifecycleNotifier<T>::~LifecycleNotifier()
+inline void LifecycleNotifier<T>::contextDestroyed()
 {
     TemporaryChange<IterationType> scope(this->m_iterating, IteratingOverAll);
     for (typename ObserverSet::iterator it = m_observers.begin(); it != m_observers.end(); it = m_observers.begin()) {
@@ -93,6 +97,7 @@ inline LifecycleNotifier<T>::~LifecycleNotifier()
         ASSERT(observer->lifecycleContext() == m_context);
         observer->contextDestroyed();
     }
+    m_context = nullptr;
 }
 
 template<typename T>