[ios] Check that CookieNotificationObserver is unregistered

Check that the observer is unregistered before destruction.
This CL changes CookieNotificationObserver to inherit from
base::CheckedObserver and check that IsInObserverList() returns false
in the destructor.

Bug: 1430392
Change-Id: If5787156edd9d61c1af2dd8335581db57535e6cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4765225
Reviewed-by: Asami Doi <asamidoi@chromium.org>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Federica Germinario <fedegermi@google.com>
Cr-Commit-Position: refs/heads/main@{#1182018}

diff --git a/ios/net/BUILD.gn b/ios/net/BUILD.gn
index 612b0d5c..4f70be5 100644
--- a/ios/net/BUILD.gn
+++ b/ios/net/BUILD.gn

@@ -31,6 +31,8 @@
     "cookies/cookie_cache.h",
     "cookies/cookie_creation_time_manager.h",
     "cookies/cookie_creation_time_manager.mm",
+    "cookies/cookie_notification_observer.h",
+    "cookies/cookie_notification_observer.mm",
     "cookies/cookie_store_ios.h",
     "cookies/cookie_store_ios.mm",
     "cookies/cookie_store_ios_client.h",