Account for ServiceWorker startup time

[igrigorik]

(based on discussion at TPAC..) /cc @slightlyoff

• The request may be blocked on SW startup and NavTiming should provide visibility into this.
• fetchStart is when we send request to worker, SW startup time should be captured before that.

How about [swStartupStart, swStartupEnd], between "Negotiate Link" and "redirect"? See processing model.

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P.S. Once we agree on the language, same logic should be replicated in Resource Timing - the worker can be killed between requests; we can't rely on SW being there. Also, UT and RT will be exposed in SW (see w3c/ServiceWorker#553), so I assume we will simply report 0's for startup time when inside a SW.

[igrigorik]

Resolved by 979980b