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This will allow them to try 10.13 first and fall back to 10.12 if there
aren't any hosts available.

Bug: 853356
Change-Id: I0fd82376ebb2c4e501a78360bbbc675c90ee7d34
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/1330693
Commit-Queue: Brad Hall <bradhall@google.com>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609743}