[PlzServiceWorker] Fix COEP update in devtools.

Coep is working properly with PlzServiceWorker, but the information was
not properly transmitted to devtools, that was still seeing it unset.
Now also pass the information to devtools when starting the worker.

Bug: 996511
Change-Id: I8e64826b42fe5aa617e52d290bf80c0de7e0cdc5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3013718


Commit-Queue: Arthur Hemery <ahemery@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#900959}