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This is a reland of 11c26e5e

Original change's description:
> Change the mac builder default to be either 10.13 or 10.15.
> 
> Once the hosts are all upgraded to 10.15 we will remove the 10.13 option.
> 
> Bug: 1121185
> Change-Id: I3177749f79ddd16b9d725693f21e79ebd752c22c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2412757
> Commit-Queue: Erik Staab <estaab@chromium.org>
> Auto-Submit: Erik Staab <estaab@chromium.org>
> Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#807218}

Bug: 1121185
Change-Id: I0b4e69122fc7b80165ca2abbd2542c24ba88d0d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2413529
Auto-Submit: Erik Staab <estaab@chromium.org>
Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Commit-Queue: Garrett Beaty <gbeaty@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807488}