GitLab

This change causes the default hermetic SDK to be version 10.13 (aka Xcode 9.3).
This also bumps the minimum build host version to 10.13.2, since that
is the earliest version on which Xcode 9.3 runs.

Bug: 780980
Change-Id: I6da3f7d3a060a22d71d5ee18bb88f51d5aedb0a2
Reviewed-on: https://chromium-review.googlesource.com/c/1394742
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#620032}