Disallow adding ContextLifecycleObservers during iteration.

Source/core/dom/ContextLifecycleObserver.h

 /*
  * Copyright (C) 2008 Apple 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 29 matching lines @@
     ExecutionContext* executionContext() const { return lifecycleContext(); }
 
     enum Type {
         GenericType,
         ActiveDOMObjectType,
     };
 
     Type observerType() const { return m_observerType; }
 
 protected:
-    explicit ContextLifecycleObserver(ExecutionContext*, Type = GenericType);
+    explicit ContextLifecycleObserver(ExecutionContext* executionContext, Type type = GenericType)
+        : LifecycleObserver(executionContext)
+        , m_observerType(type)
+    {
+    }
 
 private:
     Type m_observerType;
 };
 
 } // namespace blink
 
 #endif // ContextLifecycleObserver_h