Re: [webcomponents] Change registries to be per document and never shared between documents (#369)

(Maybe a good way to implement (1) is to check that the passed constructor's realm's global object is equal to the context object's associated window. So `otherWindow.defineElement("x-tag", XTagDefinedInDifferentGlobal)` just fails. It's not 100% but it seems like a good sanity check. Probably in addition to the proto-walking @rniwa proposed a few times.)

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