[iOS] Use CheckedObserver for request queue observers.

CheckedObserver allows for safer observation as well as allowing the
check_empty=true setting for ObserverList.

Bug: none
Change-Id: I8c74958723254a4fb07fba05206eaa0363a8caaf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1593929
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#656945}
7 files changed