Declarative attach shadow uses host's registry instead of host's node document's registry

[annevk]

What is the issue with the HTML Standard?

In whatwg/dom#1409 we changed where attachShadow() gets its initial registry from, but we didn't change how the HTML parser does it. We probably should have?

(See also #11892 for related fallout of that change.)

cc @ja-y-son @sorvell @rniwa @Jamesernator