GitLab

The latest build is expiring its single 'sizes' test:
https://ci.chromium.org/p/chrome/builders/ci/mac-chrome/3489

Likely cause most of the bots in the test pool on chrome-swarming are
moving to 10.15. This updates it to target 10.15.

Bug: None
Change-Id: I68435fc7d4c0a6f152af37edb1663802a34fbabf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2163730Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762144}