Disallow adding ContextLifecycleObservers during iteration.

Source/platform/LifecycleObserver.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 19 matching lines @@
 #include "platform/heap/Handle.h"
 #include "wtf/Assertions.h"
 
 namespace blink {
 
 template<typename T, typename Observer, typename Notifier>
 class LifecycleObserver : public WillBeGarbageCollectedMixin {
 public:
     using Context = T;
 
-    LifecycleObserver()
-        : m_lifecycleContext(nullptr)
-    {
-    }
-
 #if !ENABLE(OILPAN)
     virtual ~LifecycleObserver()
     {
         dispose();
     }
 #endif
 
     DEFINE_INLINE_VIRTUAL_TRACE()
     {
         visitor->trace(m_lifecycleContext);
     }
     virtual void contextDestroyed() { }
 
     void dispose()
     {
         setContext(nullptr);
     }
 
     Context* lifecycleContext() const { return m_lifecycleContext; }
     void clearLifecycleContext() { m_lifecycleContext = nullptr; }
 
 protected:
+    explicit LifecycleObserver(Context* context)
+        : m_lifecycleContext(nullptr)
+    {
+        setContext(context);
+    }
+
     void setContext(Context*);
 
 private:
     RawPtrWillBeWeakMember<Context> m_lifecycleContext;
 };
 
 template<typename T, typename Observer, typename Notifier>
 inline void LifecycleObserver<T, Observer, Notifier>::setContext(typename LifecycleObserver<T, Observer, Notifier>::Context* context)
 {
     if (m_lifecycleContext)
         static_cast<Notifier*>(m_lifecycleContext.get())->removeObserver(static_cast<Observer*>(this));
 
     m_lifecycleContext = context;
 
     if (m_lifecycleContext)
         static_cast<Notifier*>(m_lifecycleContext.get())->addObserver(static_cast<Observer*>(this));
 }
 
 } // namespace blink
 
 #endif // LifecycleObserver_h