GitLab

Essentially a thread-safe notifier wrapping around DnsConfigService,
intended (but not yet used) for NetworkChangeNotifier and
HostResolverManager(s) to listen to a shared object for system DNS
config changes. In subsequent CLs, this will allow us to divorce
HostResolverManager from listening to NetworkChangeNotifier and be able
to apply Chrome-internal config changes on top of the system-only
changes currently triggerring change notifications.

In non-test, will generally be expected to be broadly shared and leaked
on shutdown.  Sequence-safe destruction is still implemented (using an
internal Core class with an OnTaskRunnerDeleter) for any special cases
and for tests.

Note that some of the internal workings of the new class work similarly
to base::ObserverListThreadSafe, but it was too difficult to make that
work with the requirement that we always be able to get an initial
config once ready, whether read before or after adding observers.

Bug: 971411
Change-Id: Iac79dba9f74018ae6b9559c27df60174bf4edc93
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1654881Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Eric Orth <ericorth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672087}