Make FetchEventInit inherit from ExtendableEventInit. Fixes #683.
Make FetchEventInit inherit from ExtendableEventInit. Fixes #683.

martinthomson/ServiceWorker,justinleoye/ServiceWorker,chadoliver/ServiceWorker,chadoliver/ServiceWorker,zuzusik/ServiceWorker,justinleoye/ServiceWorker,slightlyoff/ServiceWorker,martinthomson/ServiceWorker,martinthomson/ServiceWorker,karolklp/ServiceWorker,mkruisselbrink/ServiceWorker,justinleoye/ServiceWorker,karolklp/ServiceWorker,zuzusik/ServiceWorker,zuzusik/ServiceWorker,chadoliver/ServiceWorker,mkruisselbrink/ServiceWorker,slightlyoff/ServiceWorker,karolklp/ServiceWorker