addAll() takes an array of RequestInfo objects, fetches them and adds the response objects into this Cache object.
Docs: https://developer.mozilla.org/en-US/docs/Web/API/Cache/addAll https://developers.google.com/web/updates/2015/09/updates-to-cache-api
Samples: https://googlechrome.github.io/samples/service-worker/prefetch/index.html