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We don't have enough capacity for layout tests in 10.13, and the fall
back triggering isn't working sufficiently (crbug.com/871453)

The 10.12 pool w/ 8086:0a2e GPUs has plenty of spare capacity, and
flashing them up to 10.13 isn't going to happen soon (crbug.com/873421).

Meanwhile, pending times are getting out of hand on ios-sim and
mac_chromium_rel_ng bots. So this temporarily moves layout tests
fully back to 10.12

That should free up plenty of 10.13 capacity. According to my SQL'ing,
the 10.13 pool ran ~8100 bot hours of tests yesterday; and layout tests
on mac_chromium_rel_ng accounted for ~1000 of them. (LMK if you want
my queries and I can link them.)

# No-try to help unblock mac_chromium_rel_ng
NOTRY=true

Bug: 854874
Change-Id: Ibd708f795d1df67de74cebf3f955ff175c00c0f3
Reviewed-on: https://chromium-review.googlesource.com/1175243
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583818}