Add recipe roll tryjob to build CQ as experimental.

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: "build"
 cq_status_url: "https://chromium-cq-status.appspot.com"
 svn_repo_url: "svn://svn.chromium.org/chrome/trunk/tools/build"
 
 rietveld {
   url: "https://codereview.chromium.org"
@@ skipping 16 matching lines @@
     committer_list: "project-infra-committers"
   }
 
   try_job {
     buckets {
       name: "tryserver.infra"
       builders {name: "Build Presubmit"}
       builders {name: "Build Try Recipe Test Win64"}
       builders {name: "Build Try Recipe Test Trusty64"}
       builders {name: "Build Try Recipe Test Mac"}
+      builders {
+        name: "Recipe Roll Downstream Tester"
+        experiment_percentage: 100
+      }
     }
     buckets {
       name: "chromium.buildbucket.swarming"
       builders {
         name: "build_presubmit"
         experiment_percentage: 100
       }
     }
     try_job_retry_config {
       try_job_retry_quota: 3
       global_retry_quota: 3
     }
   }
 }