[Extensions] Test SW status event dispatch for intermediate statuses.

This tests that for blink::EmbeddedWorkerStatus::STARTING and ::STOPPING
status service workers events are:

*) Acknowledged from the renderer to the browser layer
*) The extension's listener in its worker will be fired

It does this by registering an observer that fires events when these
two statuses are encountered.

Low-Coverage-Reason: service_worker_context_observer.h is an abstract
class.

Change-Id: I1df69e8e7e8276091bc0e39fd51f37e9ce4137e8
Bug: 1467015
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4846252
Auto-Submit: Justin Lulejian <jlulejian@chromium.org>
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Commit-Queue: Justin Lulejian <jlulejian@chromium.org>
Reviewed-by: David Bertoni <dbertoni@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1194409}