Events should have an initialized flag.

third_party/WebKit/Source/core/events/Event.h

 /*
  * Copyright (C) 2001 Peter Kelly (pmk@post.com)
  * Copyright (C) 2001 Tobias Anton (anton@stud.fbi.fh-darmstadt.de)
  * Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 152 matching lines @@
     virtual bool isDragEvent() const;
 
     // These events lack a DOM interface.
     virtual bool isClipboardEvent() const;
     virtual bool isBeforeTextInsertedEvent() const;
 
     virtual bool isBeforeUnloadEvent() const;
 
     bool propagationStopped() const { return m_propagationStopped || m_immediatePropagationStopped; }
     bool immediatePropagationStopped() const { return m_immediatePropagationStopped; }
+    bool wasInitialized() { return m_wasInitialized; }
 
     bool defaultPrevented() const { return m_defaultPrevented; }
     virtual void preventDefault();
     void setDefaultPrevented(bool defaultPrevented) { m_defaultPrevented = defaultPrevented; }
 
     bool defaultHandled() const { return m_defaultHandled; }
     void setDefaultHandled() { m_defaultHandled = true; }
 
     bool cancelBubble() const { return m_cancelBubble; }
     void setCancelBubble(bool cancel) { m_cancelBubble = cancel; }
@@ skipping 48 matching lines @@
     unsigned m_canBubble:1;
     unsigned m_cancelable:1;
     unsigned m_scoped:1;
     unsigned m_relatedTargetScoped:1;
 
     unsigned m_propagationStopped:1;
     unsigned m_immediatePropagationStopped:1;
     unsigned m_defaultPrevented:1;
     unsigned m_defaultHandled:1;
     unsigned m_cancelBubble:1;
+    unsigned m_wasInitialized:1;
     unsigned m_isTrusted : 1;
     unsigned m_handlingPassive : 1;
 
     unsigned short m_eventPhase;
     Member<EventTarget> m_currentTarget;
     Member<EventTarget> m_target;
     DOMTimeStamp m_createTime;
     Member<Event> m_underlyingEvent;
     Member<EventPath> m_eventPath;
     // The monotonic platform time in seconds, for input events it is the
     // event timestamp provided by the host OS and reported in the original
     // WebInputEvent instance.
     double m_platformTimeStamp;
 };
 
 #define DEFINE_EVENT_TYPE_CASTS(typeName) \
     DEFINE_TYPE_CASTS(typeName, Event, event, event->is##typeName(), event.is##typeName())
 
 } // namespace blink
 
 #endif // Event_h