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This reverts commit 6822d5b7.

Reason for revert: https://crbug.com/908551

Original change's description:
> Add 10.13 as an optional dimension for layout tests
> 
> 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}

TBR=jbudorick@chromium.org,bradhall@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 853356
Change-Id: I2d620fd3bec6b43ceda004de3925154a24986d8e
Reviewed-on: https://chromium-review.googlesource.com/c/1351503Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610950}