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Used to trigger config update in SystemDnsConfigChangeNotifier on
Android and ChromeOS where the update is triggered external to the
network stack.

Primarily needed for SystemDnsConfigChangeNotifier, but updated the
current soon-to-be-modified code in NetworkChangeNotifierPosix to use
the new unified method on DnsConfigService and removed its special
DnsConfigService overrides in order to simplify and prepare for the
transition to SystemDnsConfigChangeNotifier.

Bug: 971411
Change-Id: I944073d4ff86e43aae40d0e1404a2cfbb1bb902a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1668147Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Eric Orth <ericorth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672632}