Add builder for recipe roll tryjob.

masters/master.tryserver.infra/builders.pyl

 {
   "master_base_class": "Master4a",
   "master_classname": "InfraTryServer",
   "master_port": 21402,
   "master_port_alt": 41402,
   "slave_port": 31402,
   "templates": [
     "../master.chromium.infra/templates",
     "../master.chromium/templates",
   ],
@@ skipping 200 matching lines @@
       "recipe": "run_presubmit",
       "properties": {
         "repo_name": "recipes_py",
         "runhooks": True,
       },
       "scheduler": None,
       "slave_pools": ["trusty64_slave_pool"],
       "slavebuilddir": "presubmit_recipespy",
     },
 
+    "Recipe Roll Downstream Tester": {
+      "category": "Recipe Roll Tryjob",
+      "recipe": "infra/recipe_roll_tryjob",
+      "scheduler": None,
+      "slave_pools": ["trusty64_slave_pool"],
+      "slavebuilddir": "recipe_roll_tryjob",
+    },
+
     "gerrit-test-cq-normal": {
       "category": "Gerrit CQ",
       "recipe": "infra/gerrit_cq_test",
       "scheduler": None,
       "slave_pools": ["trusty64_slave_pool"],
       "slavebuilddir": "gerrit-test-cq-normal",
     },
   },
 
   "schedulers": {},
@@ skipping 32 matching lines @@
     "win64_slave_pool": {
       "slave_data": {
         "bits": 64,
         "os": "win",
         "version": "win7",
       },
       "slaves": ["vm253-m4"],
     },
   },
 }