Add <menuitem>, new HTMLMenuElement IDL attributes.

LayoutTests/fast/parser/parse-menuitem-expected.txt

Index: LayoutTests/fast/parser/parse-menuitem-expected.txt
diff --git a/LayoutTests/fast/parser/parse-menuitem-expected.txt b/LayoutTests/fast/parser/parse-menuitem-expected.txt
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+++ b/LayoutTests/fast/parser/parse-menuitem-expected.txt
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+This test checks parsing of menuitem tag which is a self closing tag with no end tag.
+
+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
+
+
+PASS document.getElementById('div').innerHTML is "<menuitem>"
+PASS document.getElementById('menu').innerHTML is "<menuitem id=\"first_item\"><menuitem id=\"second_item\">"
+PASS document.getElementById('first_item').firstChild is null
+PASS document.getElementById('menu').firstChild.nextSibling is document.getElementById('second_item')
+PASS successfullyParsed is true
+
+TEST COMPLETE
+