GitLab

This CL also changed from ObserverList to CallbackList since it is
much more lifecycle tolerant. It is safe for a Subscription to outlive
its associated CallbackList and destruction of a Subscription will
automatically un-register it.

Bug: 1113772
Change-Id: Ie1335f617e809b75d7b219aea48968ab54cb8225
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2401510
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Ahmed Mehfooz <amehfooz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806219}