luci: retarget mac-osxbeta-rel to Ubuntu-14.04.

TBR=lindsayw@chromium.org

Bug: 850125
Change-Id: Ie1185a8cf043d09c1bc6c8b6802115665212901a
Reviewed-on: https://chromium-review.googlesource.com/c/1259473Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596391}

infra/config/global/cr-buildbucket.cfg

@@ -2281,8 +2281,9 @@ buckets {
     builders {
       name: "mac-osxbeta-rel"
       mixins: "fyi-ci"
-      mixins: "mac"
-      dimensions:"os:Mac-10.14"
+      # This is launching & collecting entirely isolated tests.
+      # OS shouldn't matter.
+      dimensions: "os:Ubuntu-14.04"
     }
     builders {
       name: "mac-views-rel"