When the service worker is registered for the first time, resources loaded
before it becomes active won't go through the service worker. These resources
may be served from the browser's HTTP cache on subsequent requests, bypassing
the service worker cache entirely.
The new `preload` option accepts a regex pattern. On first visit (when no
controller exists), it waits for the service worker to take control, then
sends a message with URLs from `performance.getEntriesByType("resource")`
that match the pattern. The service worker fetches and caches these resources.
Rosa Gutierrez
089fcf94f8