S13nServiceWorker: Set more accurate worker ready time for subresource loading

Before this CL we recorded worker ready time just after getting a
mojo pointer to the controller service worker, but getting the
mojo pointer doesn't guarantee that the controller service worker
is actually running. This means that the recorded timing didn't
capture worker startup delay. To have more accurate timing,
use |dispatch_event_time| of ServiceWorkerFetchEventTiming as
worker ready time. |dispatch_event_time| is recorded just before
dispatching a fetch event so this is reasonable.

Cq-Include-Trybots: luci.chromium.try:linux_mojo
Bug: 877972
Change-Id: Ifb8e905095794770dd8a4973e1e0d569ccfde00e
Reviewed-on: https://chromium-review.googlesource.com/c/1278459
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599587}
3 files changed