ServiceWorker

4b5d0413 - Maintain timing for worker start/ready, for navigation requests. (#1575)

Maintain timing for worker start/ready, for navigation requests. (#1575) * Maintain timing for worker start/ready, for navigation requests. The timing when worker is started or ready are web-accessible via the navigation timing API (https://w3c.github.io/navigation-timing/). `workerStart` is specified to be the time when the service worker is run. In Chromium and Gecko, `worker ready time` (exposed as `fetchStart`) is set to the right before the time when the Fetch event can be dispatched for the newly created worker. In preparation for https://github.com/w3c/navigation-timing/issues/136