Verify request's window usage for preloads

[Anne van Kesteren]

What is the issue with the Fetch Standard?

I see we are using request's window for a number of things here, but I wonder if that is correct.

In particular a forwarded request in a service worker can have its client set to the service worker environment, but its window would be the document environment where the request originated.

This also seems incompatible with allowing preloads to work in workers as has at least been envisioned.

@noamr I hope you can take a look at this.