Intent to implement and ship PerformanceResourceTiming.workerStart

[Ben Kelly]

The PerformanceResourceTiming API has had a `workerStart` value specified
for a couple years now. Its defined to represent the time when we trigger
a service worker FetchEvent. It will be zero if service workers are not
involved.

https://bugzilla.mozilla.org/show_bug.cgi?id=1191943
https://w3c.github.io/resource-timing/#dom-performanceresourcetiming-workerstart

This will be available on all platforms.

I'm targeting it for release in FF58.

It is not behind a pref. The property will always be there. If SW are
disabled then the value will always be zero.

We have some work going on to try to display service worker timing
information in devtools:

https://bugzilla.mozilla.org/show_bug.cgi?id=1353798

Chrome has implemented workerStart for a while and there is a WPT test.

There are some spec/compat questions to clarify:

https://github.com/w3c/resource-timing/issues/118
https://github.com/w3c/resource-timing/issues/119

But I don't think those need to block shipping the property. We can adjust
the values as the spec is clarified. Also, one of the reasons I am
implementing this now is so we can enable the WPT test. Our
PerformanceResourceTiming code is completely broken for SW in e10s right
now and we missed it because the test is not passing. (This will be fixed
in FF58 as well.)

The WPT test is here and is being expanded slightly with our implementation:

https://searchfox.org/mozilla-central/source/testing/web-platform/tests/service-workers/service-worker/resource-timing.https.html

Please let me know if there are questions or concerns. Thanks.

Ben