[Extensions] Skip killing renderer in certain situations.

Before this change we would kill the renderer with a bad message if the
result of finishing the external request was anything other than
content::ServiceWorkerExternalRequestResult::kOk. There's evidence that
content::ServiceWorkerExternalRequestResult::kWorkerNotFound and
content::ServiceWorkerExternalRequestResult::kWorkerNotRunning are
occurring frequently. It seems likely that there is a race with
`worker_stopped` which causes it to not be accurate and so we don't
return early.

After this change we will not kill the renderer for these two statuses.
We don't expect any behavioral change except to avoid crashing the
(service worker-based) extension renderer.

This is a quick fix to prevent a release delay.

Change-Id: Ic9105058b94be7074a121f850f3b1c44a254fd74
Bug: 1485425
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4953976
Reviewed-by: David Bertoni <dbertoni@chromium.org>
Auto-Submit: Justin Lulejian <jlulejian@chromium.org>
Commit-Queue: Justin Lulejian <jlulejian@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1212297}