Implement contextmenu attribute with basic support of <menu>

Source/core/events/Event.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 149 matching lines @@
 bool Event::isGestureEvent() const
 {
     return false;
 }
 
 bool Event::isWheelEvent() const
 {
     return false;
 }
 
+bool Event::isRelatedEvent() const
+{
+    return false;
+}
+
 bool Event::isDragEvent() const
 {
     return false;
 }
 
 bool Event::isClipboardEvent() const
 {
     return false;
 }
 
@@ skipping 68 matching lines @@
 
 void Event::trace(Visitor* visitor)
 {
     visitor->trace(m_currentTarget);
     visitor->trace(m_target);
     visitor->trace(m_underlyingEvent);
     visitor->trace(m_eventPath);
 }
 
 } // namespace blink