Implement contextmenu attribute with basic support of <menu>

Source/core/html/HTMLMenuItemElement.idl

Index: Source/core/html/HTMLMenuItemElement.idl
diff --git a/Source/core/html/HTMLMenuItemElement.idl b/Source/core/html/HTMLMenuItemElement.idl
new file mode 100644
index 0000000000000000000000000000000000000000..a44cb611321c4e83b75f7975f8e4a507b81ad9c4
--- /dev/null
+++ b/Source/core/html/HTMLMenuItemElement.idl
@@ -0,0 +1,13 @@
+// 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.
+
+[
+    RuntimeEnabled=MenuElement,
+] interface HTMLMenuItemElement : HTMLElement {
+    [Reflect] readonly attribute DOMString type;
+    [Reflect] readonly attribute DOMString label;
+    [Reflect] readonly attribute boolean disabled;
+    [Reflect] readonly attribute boolean checked;
+    [Reflect] readonly attribute boolean default;
+};