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Add the DnsLatency routine to the ServiceConnection.

BUG=chromium:956783
TEST=1) chromeos_unittests --gtest_filter=CrosHealthdServiceConnectionTest.*
2) unit_tests --gtest_filter=DeviceCommandRunRoutineJobTest*
3) Applied DnsLatency changes and successfully ran the
DnsLatency routine on a DUT (verified using cros-health-tool diag
--action=run_routine --routine=dns_latency).

Change-Id: I2e00fae7f641344a20372923c4d6be8e51cc5a2e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2345004
Commit-Queue: Kartik Hegde <khegde@chromium.org>
Reviewed-by: Paul Moy <pmoy@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819632}