infra: Add "cores:4" for Mac Goma RBE ToT builder

Bug: 915568
Change-Id: I917d801b38c8afd1ac29af67bdd9d704e53514d9
Reviewed-on: https://chromium-review.googlesource.com/c/1491739
Auto-Submit: Simon Que <sque@chromium.org>
Reviewed-by: smut <smut@google.com>
Commit-Queue: smut <smut@google.com>
Cr-Commit-Position: refs/heads/master@{#636275}

infra/config/cr-buildbucket.cfg

@@ -2588,6 +2588,7 @@ buckets {
     builders {
       name: "Chromium Mac Goma RBE ToT"
       dimensions: "os:Mac-10.13"
+      dimensions: "cores:4"
       mixins: "goma-ci"
       mixins: "experimental"
     }