[Extensions] Introduce ExtensionHostTestHelper

Introduce a new test class, ExtensionHostTestHelper, to listen for
specific ExtensionHost-related events. This will be used in migrating
over from the legacy NOTIFICATION_-style observers in tests that
currently use content::WindowedNotificationObserver.

This first CL adds the ExtensionHostTestHelper class and boilerplate,
along with instrumentation to watch for an ExtensionHost being
destroyed. Adding more events should be straightforward.

This class should also be able to be leveraged in (or directly replace)
some of our other test helpers that watch for background page events
once we add some more filtering criteria (e.g. host type) and events.

Bug: 1174738
Change-Id: I7cc2ef5c5b36ebc3a3030c8b0888197884dabd3a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3154739
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: David Bertoni <dbertoni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#921489}