[xhr] Don't use "JavaScript global environment" (#33)

The usages here are much simpler than those in HTML, it seems. You can e.g. replace

> If the JavaScript global environment is a document environment

with

> If the entry settings object's global object is a Window

and

> If the JavaScript global environment is a worker environment 

with

> If the entry settings object's global object is not a Window

Hmm, I guess HTML can do that too, brb... It's much easier once you realize that it's not possible to access a worker-version of the constructor from a window, and vice-versa.

---
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/xhr/issues/33