Moved from w3c/ServiceWorker#1518

https://html.spec.whatwg.org/multipage/webappapis.html#fetch-a-classic-script, https://html.spec.whatwg.org/multipage/webappapis.html#fetch-a-module-worker-script-tree, and https://html.spec.whatwg.org/multipage/workers.html#dom-workerglobalscope-importscripts accept "perform the fetch" steps but it should be clear it's an official optional parameter.