Flip builders to use builder_group property instead of mastername.

Bug: crbug.com/1109276
Change-Id: Ib4104a6d2eb9afe4af7ff88a217c01b07860e316
diff --git a/tools/luci/config/cr-buildbucket.cfg b/tools/luci/config/cr-buildbucket.cfg
index 68dcc97..daecd60 100644
--- a/tools/luci/config/cr-buildbucket.cfg
+++ b/tools/luci/config/cr-buildbucket.cfg
@@ -45,7 +45,7 @@
         cipd_package: "infra/recipe_bundles/chromium.googlesource.com/chromium/tools/build"
         cipd_version: "refs/heads/master"
         name: "art"
-        properties: "mastername:client.art"
+        properties: "builder_group:client.art"
       }
     }