Convert service_discovery_client_mac_unittest to use ARC

See https://chromium.googlesource.com/chromium/src/+/main/docs/mac/arc.md
for information about this conversion.

Bug: 1280317
Change-Id: I765308cd92652715107e6dcafd45edf97b03db6e
Include-Ci-Only-Tests: true
Validate-Test-Flakiness: skip
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4567143


Commit-Queue: Leonard Grey <lgrey@chromium.org>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Auto-Submit: Avi Drissman <avi@chromium.org>
Reviewed-by: Leonard Grey <lgrey@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1149136}