Require MacOS 10.13 for 'Mac Builder'.

R=tandrii

No-Try: True
Bug: 872448
Change-Id: Ia12df35141be3e9073167f24bd70fdf02568c66e
Reviewed-on: https://chromium-review.googlesource.com/1184249
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584921}

infra/config/global/cr-buildbucket.cfg

@@ -1188,6 +1188,7 @@ buckets {
     builders {
       name: "Mac Builder"
       mixins: "mac-ci"
+      dimensions:"os:Mac-10.13"
     }
     builders {
       name: "Mac10.10 Tests"