[nav+resource-timing] add workerStart to measure ServiceWorker startup

If the app is ServiceWorker-enabled [1] then all fetches are routed to the
SW, where they may be fulfilled from local cache, rewritten, forwarded to
remote server, etc. However, on navigation the worker may not be around and
will need to be started before it can service the request; later the worker
may be killed and will need to be (re)started to service a subresource
request.

The application needs to be able to distinguish and measure the worker
startup time.

To address this, I'd like to propose that we add "workerStart" attribute to
Nav+Resource Timing:
https://github.com/w3c/resource-timing/issues/11

ig

[1] https://slightlyoff.github.io/ServiceWorker/spec/service_worker/