Implement contextmenu attribute with basic support of <menu>

Source/core/events/RelatedEvent.idl

Index: Source/core/events/RelatedEvent.idl
diff --git a/Source/core/events/RelatedEvent.idl b/Source/core/events/RelatedEvent.idl
new file mode 100644
index 0000000000000000000000000000000000000000..f693ed3f6a232d158d1c2f18f32e90ab9795bcbe
--- /dev/null
+++ b/Source/core/events/RelatedEvent.idl
@@ -0,0 +1,11 @@
+// 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.
+
+[
+    EventConstructor,

[tkent, 2014/07/31 06:07:55]

Need a layout test for constructor.

[pals, 2014/08/01 10:03:07]

Rebased LayoutTests/webexposed/global-constructors-listing.html.
Do you mean the same?

[tkent, 2014/08/04 02:26:12]

No.  global-constructors-list.html just lists constructors.  We need a test of
RelatedEvent constructor behavior. See LayoutTests/fast/event/constructors/.

[pals, 2014/08/04 08:58:26]

Added tests. 

+    RuntimeEnabled=ContextMenu,
+] interface RelatedEvent : Event {
+    [InitializedByEventConstructor] readonly attribute EventTarget relatedTarget;
+};
+