[Extensions] Remove NOTIFICATION_EXTENSION_HOST_CREATED

The legacy NOTIFICATION_-style events are deprecated.
NOTIFICATION_EXTENSION_HOST_CREATED can be replaced with
ExtensionHostRegistry::Observer::OnExtensionHostCreated().

Remove NOTIFICATION_EXTENSION_HOST_CREATED entirely, converting all
usages to the new observer method.

Modify ExtensionHostTestHelper to also return the corresponding
ExtensionHost from its "WaitFor" methods if and only if the host is
still valid. This also fixes a subtle potential (test-only) UAF bug
where a test could try to access a host from a
WindowedNotificationObserver even if that host was synchronously
destroyed from creation.

Bug: 1174737
Change-Id: I7586b069fbe5d1d9d77c6791bb4b3d79fcc5aa7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3160293
Reviewed-by: David Bertoni <dbertoni@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#922588}