InitEvent should not do anything if dispatch flag is set.

third_party/WebKit/Source/core/events/MutationEvent.cpp

 /*
  * 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, 2005, 2006, 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 31 matching lines @@
 }
 
 MutationEvent::~MutationEvent()
 {
 }
 
 void MutationEvent::initMutationEvent(const AtomicString& type, bool canBubble, bool cancelable, Node* relatedNode,
                                       const String& prevValue, const String& newValue,
                                       const String& attrName, unsigned short attrChange)
 {
-    if (dispatched())
+    if (isBeingDispatched())
         return;
 
     initEvent(type, canBubble, cancelable);
 
     m_relatedNode = relatedNode;
     m_prevValue = prevValue;
     m_newValue = newValue;
     m_attrName = attrName;
     m_attrChange = attrChange;
 }
 
 const AtomicString& MutationEvent::interfaceName() const
 {
     return EventNames::MutationEvent;
 }
 
 DEFINE_TRACE(MutationEvent)
 {
     visitor->trace(m_relatedNode);
     Event::trace(visitor);
 }
 
 } // namespace blink