cq: start using LUCI downstream testers as an experiment.

R=hinoka

Bug: 830992
Change-Id: Ibb0a269c10d2939505e4286e3278efb650d82636
Reviewed-on: https://chromium-review.googlesource.com/1024869Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

infra/config/cq.cfg

@@ -23,6 +23,18 @@ verifiers {
       builders { name: "Skia downstream Recipe Roll tester from Depot Tools" }
       builders { name: "Skia Buildbot downstream Recipe Roll tester from Depot Tools" }
     }
+    buckets {
+      name: "luci.infra-internal.try"
+
+      builders {
+        name: "build_limited Roll Tester (depot_tools)"
+        experiment_percentage: 100
+      }
+      builders {
+        name: "release_scripts Roll Tester (depot_tools)"
+        experiment_percentage: 100
+      }
+    }
 
     buckets {
       name: "master.internal.infra.try"