Do not use untraced members while notifying ActiveDOMObjects.

third_party/WebKit/Source/core/dom/ContextLifecycleNotifier.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 40 matching lines @@
     // Need a default constructor to link core and modules separately.
     // If no default constructor, we will see an error: "constructor for
     // 'blink::ExecutionContext' must explicitly initialize the base class
     // 'blink::ContextLifecycleNotifier' which does not have a default
     // constructor ExecutionContext::ExecutionContext()".
     ContextLifecycleNotifier() { }
 
 #if DCHECK_IS_ON()
     bool contains(ActiveDOMObject*) const;
 #endif
+
+private:
+    // Iterates over a snapshot of the observers, invoking the
+    // callback with each observer that is an ActiveDOMObject.
+    // Observers must not be added during |eachActiveDOMObject|. It is
+    // safe to remove observers, though. The callback is not invoked
+    // for observers which get removed first.
+    void eachActiveDOMObject(void (*)(ActiveDOMObject*));
 };
 
 } // namespace blink
 
 #endif // ContextLifecycleNotifier_h