[ServiceWorker] Create a synchronous stopping observer method.

This method will allow extensions to get an early notice that a worker
will stop. This will allow extensions to no longer assume a worker is
running and ready to receive IPCs (extension events) when the worker is
stopping (and wouldn't necessarily be ready for them anyway).

Bug: 331358156
Change-Id: Ifce9a835e2899d97e2a70b0462eefdd9b83c3cc1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6072243
Commit-Queue: Justin Lulejian <jlulejian@chromium.org>
Auto-Submit: Justin Lulejian <jlulejian@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1392327}