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This will limit Magic Tether hosts to devices that have gone through
Better Together Setup.

Bug: 870072
Change-Id: I9fe120f4b8d07f831ebe84aeb28267ff379f2e82
Reviewed-on: https://chromium-review.googlesource.com/1180473Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Jeremy Klein <jlklein@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584617}