[navigation/resource-timing] added workerStart to measure ServiceWorker startup

Landed updates to Nav + Resource Timing that add new "workerStart"
attribute. If the page is using ServiceWorker, the UA may need to first
start (or restart) the worker before initiating the fetch request, and the
delta between [workerStart, fetchStart] can be used to measure this time.

- https://github.com/w3c/resource-timing/pull/18
-
https://github.com/w3c/navigation-timing/commit/979980b4646ff0cf5c2bd4240ad098699bb1123b


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