Sites trying to analyze the performance of a service worker need to understand what is happening between when the FetchEvent is fired and the FetchEvent.respondWith() promise is resolved.  One common activity for service workers is to call `cache.match()` and use the resulting Response to fulfill the `respondWith()` promise.

Let's investigate adding `.cacheMatchStart` and `.cacheMatchEnd` for tracking these values.  This will work as long as the cache-provided Response is passed to `respondWith()` without the service worker re-packing it into a different Response, etc.

This will require updating the spec, WPT, and getting agreement from other browsers.