Change <menuitem> parsing rules to match spec

This CL changes <menuitem> parsing rules so that they match the current spec.
<menuitem> is no more a self closing tag.

The link below shows the latest change made to the spec.
whatwg/html#2319

BUG=412945

Review-Url: https://codereview.chromium.org/2819773002
Cr-Commit-Position: refs/heads/master@{#464906}

Author: yuzus

html/syntax/serializing-html-fragments/serializing.html

@@ -178,7 +178,7 @@
 
 var void_elements = [
   "area", "base", "basefont", "bgsound", "br", "col", "embed",
-  "frame", "hr", "img", "input", "keygen", "link", "menuitem",
+  "frame", "hr", "img", "input", "keygen", "link",
   "meta", "param", "source", "track", "wbr"
 ];