web: Support named access on document for polyfill elements

[danielhjacobs]

We have now reached territory of how much we value compatibility over code complexity. This mimics behavior defined in https://html.spec.whatwg.org/multipage/nav-history-apis.html#named-access-on-the-window-object. I designed this to see how complex it would make the code. The answer, as it turns out, is fairly.

embed, form, img, or object elements that have a name content attribute whose value is name and are in a document tree with window's associated Document as their root;

See also #22442