Implement contextmenu attribute with basic support of <menu>

Source/core/events/RelatedEvent.h

Index: Source/core/events/RelatedEvent.h
diff --git a/Source/core/events/RelatedEvent.h b/Source/core/events/RelatedEvent.h
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+++ b/Source/core/events/RelatedEvent.h
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+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef RelatedEvent_h
+#define RelatedEvent_h
+
+#include "core/events/Event.h"
+
+namespace blink {
+
+struct RelatedEventInit : public EventInit {
+    RelatedEventInit();
+    RefPtrWillBeMember<EventTarget> relatedTarget;
+};
+
+class RelatedEvent FINAL : public Event {
+public:
+    static PassRefPtrWillBeRawPtr<RelatedEvent> create();
+    static PassRefPtrWillBeRawPtr<RelatedEvent> create(const AtomicString& eventType);
+    static PassRefPtrWillBeRawPtr<RelatedEvent> create(const AtomicString& eventType, const RelatedEventInit&);
+
+    virtual ~RelatedEvent();
+
+    EventTarget* relatedTarget() const { return m_relatedTarget.get(); }
+    void setRelatedTarget(PassRefPtrWillBeRawPtr<EventTarget> relatedTarget) { m_relatedTarget = relatedTarget; }
+
+
+    virtual const AtomicString& interfaceName() const OVERRIDE { return EventNames::RelatedEvent; }
+    virtual bool isRelatedEvent() const OVERRIDE { return true; }
+
+    virtual void trace(Visitor*) OVERRIDE;
+
+private:
+    RelatedEvent();
+    explicit RelatedEvent(const AtomicString& type);
+    RelatedEvent(const AtomicString& type, const RelatedEventInit&);
+
+    RefPtrWillBeMember<EventTarget> m_relatedTarget;
+};
+
+DEFINE_EVENT_TYPE_CASTS(RelatedEvent);
+
+} // namespace blink
+
+#endif // RelatedEvent_h