[Extensions] Hoist observer_ to ExtensionPlatformBrowserTest

ExtensionBrowserTest has a ChromeExtensionTestNotificationObserver
member, `observer_`, which is used to wait for various extension-
related events. ChromeExtensionTestNotificationObserver can't be
included in the experimental desktop android build yet, but the
parent class, ExtensionTestNotificationObserver, can.

Add a `test_notification_observer_` member on
ExtensionPlatformBrowserTest that's implemented as an
ExtensionTestNotificationObserver by default, and overridden to the
ChromeExtensionTestNotificationObserver in ExtensionBrowserTest. This
allows us to move `ExtensionBrowserTest::WaitForExtensionViewsToLoad()`
up to ExtensionPlatformBrowserTest and will enable more methods to move
up in the future.

This CL has no effective behavior change.

Bug: 404581990
Change-Id: I252419e1ac891e993ba74df3ca922376ab7704aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6435314
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Devlin Cronin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1443826}