cq.cfg: add experimental swarmbucket builder

R=estaab@chromium.org, vadimsh@chromium.org
TBR=tandrii@chromium.org
BUG=629676

Review URL: https://codereview.chromium.org/2177713002 .

infra/config/OWNERS

 set noparent
 iannucci@chromium.org
+nodir@chromium.org
 phajdan.jr@chromium.org
 sergiyb@chromium.org
 tandrii@chromium.org

infra/config/cq.cfg

@@ -23,6 +23,10 @@ verifiers {
       builders {
         name: "Depot Tools Presubmit"
       }
+      builders {
+        name: "Depot Tools Presubmit (Swarming)"
+        experiment_percentage: 100
+      }
       builders {
         name: "Recipe Roll Downstream Tester"
         experiment_percentage: 100