Implement 'mouseenter' and 'mouseleave' from DOM Level 3 Events.

Source/core/dom/Element.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Peter Kelly (pmk@post.com)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2013 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
@@ skipping 190 matching lines @@
     DEFINE_ATTRIBUTE_EVENT_LISTENER(drop);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(dragstart);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(drag);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(dragend);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(input);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(invalid);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(keydown);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(keypress);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(keyup);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(mousedown);
+    DEFINE_ATTRIBUTE_EVENT_LISTENER(mouseenter);
+    DEFINE_ATTRIBUTE_EVENT_LISTENER(mouseleave);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(mousemove);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(mouseout);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(mouseover);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(mouseup);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(mousewheel);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(scroll);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(select);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(submit);
 
     // These four attribute event handler attributes are overridden by HTMLBodyElement
@@ skipping 840 matching lines @@
 
 inline const Attribute* ElementData::attributeItem(unsigned index) const
 {
     RELEASE_ASSERT(index < length());
     return attributeBase() + index;
 }
 
 } // namespace
 
 #endif