Reland "Switch mac_chromium_compile_dbg_ng to builderless"

This is a reland of bd916257

Original change's description:
> Switch mac_chromium_compile_db_ng to builderless
>
> Bug: 999206
> Change-Id: I91c01200552c4170ac6d18badd5bdd113396526d
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1792461
> Commit-Queue: Stephen Martinis <martiniss@chromium.org>
> Commit-Queue: John Budorick <jbudorick@chromium.org>
> Auto-Submit: Stephen Martinis <martiniss@chromium.org>
> Reviewed-by: John Budorick <jbudorick@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#694874}

Bug: 999206, 956776
Change-Id: I9fa138a8fa53073b46d69c0322ada905533f8374
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1796630
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695786}

infra/config/cr-buildbucket.cfg

@@ -4768,8 +4768,10 @@ buckets {
     builders {
       mixins: "mac-try"
       mixins: "goma-j150"
+      mixins: "builderless"
       name: "mac_chromium_compile_dbg_ng"
       dimensions:"os:Mac-10.13"
+      dimensions:"ssd:1"
     }
     builders { mixins: "mac-try" name: "mac_chromium_compile_rel_ng" }
     builders { mixins: "mac-try" name: "mac_chromium_dbg_ng" }