Maintain timing for worker start/ready, for navigation requests. (#1575)

* Maintain timing for worker start/ready, for navigation requests.

The timing when worker is started or ready are web-accessible via
the navigation timing API (https://w3c.github.io/navigation-timing/).

`workerStart` is specified to be the time when the service worker is run.

In Chromium and Gecko, `worker ready time` (exposed as `fetchStart`)
is set to the right before the time when the Fetch event can be
dispatched for the newly created worker.

In preparation for w3c/navigation-timing#136

docs/index.bs

@@ -2985,6 +2985,7 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
 
       : Input
       :: |request|, a [=/request=]
+      :: |useHighResPerformanceTimers|, a boolean
       : Output
       :: |response|, a [=/response=]
 
@@ -2998,6 +2999,7 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
       1. Let |preloadResponse| be a new [=promise=].
       1. Let |workerRealm| be null.
       1. Let |eventHandled| be null.
+      1. Let |timingInfo| be a new [=service worker timing info=].
       1. Let |fetchInstance| be the instance of the [=/fetch=] algorithm representing the ongoing fetch.
       1. Assert: |request|'s [=request/destination=] is not "<code>serviceworker</code>".
       1. If |request|'s [=request/destination=] is either "<code>embed</code>" or "<code>object</code>", then:
@@ -3046,6 +3048,8 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
       1. If the result of running the [=Should Skip Event=] algorithm with "fetch" and |activeWorker| is true, then:
           1. If |shouldSoftUpdate| is true, then [=in parallel=] run the [=Soft Update=] algorithm with |registration|.
           1. Return null.
+      1. If |useHighResPerformanceTimers| is true, then set |useHighResPerformanceTimers| to |activeWorker|'s [=service worker/global object=]'s [=WorkerGlobalScope/cross-origin isolated capability=].
+      1. Let |timingInfo|'s [=service worker timing info/start time=] be the [=coarsened shared current time=] given |useHighResPerformanceTimers|.
       1. If |activeWorker|'s <a>state</a> is "`activating`", wait for |activeWorker|'s <a>state</a> to become "`activated`".
       1. If the result of running the [=Run Service Worker=] algorithm with |activeWorker| is *failure*, then set |handleFetchFailed| to true.
       1. Else:
@@ -3062,6 +3066,7 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
               1. If |request| is a <a>non-subresource request</a> and |request|'s [=request/destination=] is not {{RequestDestination/"report"}}, initialize |e|'s {{FetchEvent/resultingClientId}} attribute to |reservedClient|'s [=environment/id=], and to the empty string otherwise.
               1. If |request| is a <a>navigation request</a>, initialize |e|'s {{FetchEvent/replacesClientId}} attribute to |request|'s [=request/replaces client id=], and to the empty string otherwise.
               1. Initialize |e|’s {{FetchEvent/handled}} to |eventHandled|.
+              1. Let |timingInfo|'s [=service worker timing info/fetch event dispatch time=] to the [=coarsened shared current time=] given |useHighResPerformanceTimers|.
               1. <a>Dispatch</a> |e| at |activeWorker|'s [=service worker/global object=].
               1. Invoke [=Update Service Worker Extended Events Set=] with |activeWorker| and |e|.
               1. If |e|'s [=FetchEvent/respond-with entered flag=] is set, set |respondWithEntered| to true.
@@ -3072,6 +3077,7 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
               1. If |response| is null, |request|'s [=request/body=] is not null, and |request|'s [=request/body=]'s [=Body/source=] is null, then:
                   1. If |request|'s [=request/body=] is [=Body/unusable=], set |handleFetchFailed| to true.
                   1. Else, [=ReadableStream/cancel=] |request|'s [=request/body=] with undefined.
+              1. If |response| is not null, then set |response|'s [=response/service worker timing info=] to |timingInfo|.
               1. If |e|'s <a>canceled flag</a> is set, set |eventCanceled| to true.
               1. If |fetchInstance| is [=fetch/terminated=], then [=queue a task=] to [=AbortSignal/signal abort=] on |requestObject|'s {{Request/signal}}.