Redo top-level origin and add top-level creation URL

An environment's top-level origin is null during the initial navigation (before the response arrived) and otherwise represents the origin of the top-level document. It is currently implementation-defined for non-dedicated workers, but hopefully that can be sorted soon.

An environment's top-level creation URL is the URL of the top-level document. It is null for workers as they do not need the concept.

Needed for whatwg/fetch#943 and whatwg#5558.