Redefine report delivery in terms of Fetch

[miketaylr]

https://w3c.github.io/network-error-logging/#report-delivery

In particular, we should be able to call fetch explicitly inside the Generate / Deliver a Network Error Report algorithms. Also elapsed_time should use fetch time machinery.

[miketaylr]

https://fetch.spec.whatwg.org/#fetch-elsewhere-request will be useful to review for this.