CQ: add mac_chromium_10.12_rel_ng to 5% experiment for LUCI migration.

TBR=maruel@chromium.org

Bug: 731360
Change-Id: I189644631ee8781f8590bc3dce95abf6849b766f
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/982099
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546116}

infra/config/branch/cq.cfg

@@ -153,6 +153,14 @@ verifiers {
         name: "mac_chromium_rel_ng"
         equivalent_to { bucket: "luci.chromium.try" percentage: 0 }
       }
+      builders {
+        # The only purpose of this builder here is get data for migrating this
+        # builder to LUCI, see https://crbug.com/731360.
+        # TODO(tandrii): remove this builder once enough data has been
+        # collected.
+        name: "mac_chromium_10.12_rel_ng"
+        experiment_percentage: 5
+      }
       builders {
         # The only purpose of this builder here is get data for migrating this
         # builder to LUCI, see https://crbug.com/807149.