[w3c/webcomponents] Should :defined match <x-foo> in browsing-context-less documents? (#540)

Originally https://bugs.chromium.org/p/chromium/issues/detail?id=635402

In documents without a browsing context, the whole custom elements machinery is offline. So `<x-foo>` is equivalent to `<asdf>`: both are never going to become customized. They are just simple unknown elements.

Currently step 7.3 of ["create an element"](https://dom.spec.whatwg.org/#concept-create-element) recognizes this and does not mark `<x-foo>` as "undefined". So in such documents, `:defined` will match `<x-foo>`, even though in the main document with a browsing context, `:defined` will not match `<x-foo>`.

Probably nobody will encounter this in practice.

I think the spec makes sense as-is. But the consequences are a bit unintuitive. So if anyone has opinions on this let me know and we can certainly change the spec.

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