cq.cfg: add experimental swarmbucket builder

infra/config/cq.cfg

 # Commit Queue configuration file. The documentation of the format can be found
 # at http://luci-config.appspot.com/schemas/projects/refs:cq.cfg.
 
 version: 1
 cq_name: "depot_tools"
 cq_status_url: "https://chromium-cq-status.appspot.com"
 git_repo_url: "https://chromium.googlesource.com/chromium/tools/depot_tools"
 
 gerrit {}
 rietveld {
   url: "https://codereview.chromium.org"
 }
 
 verifiers {
   reviewer_lgtm {
     committer_list: "project-infra-committers"
     dry_run_access_list: "project-infra-tryjob-access"
   }
 
   try_job {
     buckets {
       name: "master.tryserver.infra"
       builders {
         name: "Depot Tools Presubmit"
       }
       builders {
+        name: "Depot Tools Presubmit (Swarming)"
+        experiment_percentage: 100
+      }
+      builders {
         name: "Recipe Roll Downstream Tester"
         experiment_percentage: 100
       }
     }
   }
 }