Update: legend to provide name to optgroup

[scottaohara]

In regards to the updated content model for the select element and its allowed children, an optgroup can have a legend element as its first child, and this legend needs to be able to name the optgroup similarly to how a legend names a fieldset.

see: https://github.com/whatwg/html/pull/10586

WPT - https://github.com/web-platform-tests/wpt/pull/49645