Add mac-osxbeta-rel trybot

Bug: 999206
Change-Id: I60ab32a236575c4d9b65361e487f73f840405fac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1776605Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695855}

infra/config/cr-buildbucket.cfg

@@ -4739,6 +4739,13 @@ buckets {
       mixins: "goma-rbe-prod"
       name: "gpu-try-mac-intel-dbg"
     }
+    builders {
+      name: "mac-osxbeta-rel"
+      mixins: "mac-try"
+      mixins: "builderless"
+      dimensions: "os:Mac-10.13"
+      dimensions: "ssd:1"
+    }
     builders {
       mixins: "mac-angle-try"
       mixins: "goma-rbe-prod"

infra/config/luci-milo.cfg

@@ -4766,6 +4766,9 @@ consoles {
   builders {
     name: "buildbucket/luci.chromium.try/ios-slimnav"
   }
+  builders {
+    name: "buildbucket/luci.chromium.try/mac-osxbeta-rel"
+  }
   builders {
     name: "buildbucket/luci.chromium.try/mac-rel"
   }