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.
https://github.com/whatwg/html/pull/2319

BUG=412945

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