Disallow adding ContextLifecycleObservers during iteration.

Source/core/frame/FrameDestructionObserver.h

 /*
  * Copyright (C) 2011 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 20 matching lines @@
 #include "platform/LifecycleObserver.h"
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 class LocalFrame;
 class LocalFrameLifecycleNotifier;
 
 class CORE_EXPORT FrameDestructionObserver : public LifecycleObserver<LocalFrame, FrameDestructionObserver, LocalFrameLifecycleNotifier> {
 public:
-    virtual void willDetachFrameHost();
+    virtual void willDetachFrameHost() { }
     virtual void contextDestroyed()
     {
         setContext(nullptr);
     }
 
     LocalFrame* frame() const { return lifecycleContext(); }
 
 protected:
-    explicit FrameDestructionObserver(LocalFrame*);
+    explicit FrameDestructionObserver(LocalFrame* frame)
+        : LifecycleObserver(frame)
+    {
+    }
 };
 
 } // namespace blink
 
 #endif // FrameDestructionObserver_h