Bug 246069

In https://github.com/whatwg/fetch/pull/1343, the Http-network fetch and fetch() method have been updated to use AbortSignal's abort reason (or a serialized version of it) when erroring streams and aborting the fetch() call, rather than unconditionally using an AbortError. This change also includes using the abort reason to abort the fetch controller.

There are some related changes in the Service Worker spec in https://github.com/w3c/ServiceWorker/pull/1655. When a service worker intercepts a fetch request, the signal's abort reason is passed to the service worker to abort the fetch controller and signal abort in the Handle Fetch algorithm.

The same change needs to be made to the Safari implementation.