Persistent ServiceWorker

https://guest271314.github.io/persistent-serviceworker/ (EventSource, FetchEvent)

References:

• https://bugs.chromium.org/p/chromium/issues/detail?id=1152255
• https://bugs.chromium.org/p/chromium/issues/detail?id=1189678
• w3c/ServiceWorker#980
• w3c/ServiceWorker#1558
• w3c/ServiceWorker#1624