Switch builders to use builder_group property instead of mastername.

Bug: crbug.com/1109276
Change-Id: Ie040e0d923e8b3836e69759cb8f798af062bcf78
diff --git a/infra/config/global/cr-buildbucket.cfg b/infra/config/global/cr-buildbucket.cfg
index 87925c3..4fc504a 100644
--- a/infra/config/global/cr-buildbucket.cfg
+++ b/infra/config/global/cr-buildbucket.cfg
@@ -102,7 +102,7 @@
       swarming_tags: "vpython:native-python-wrapper"
       build_numbers: YES
       recipe {
-        properties: "mastername:internal.client.r8"
+        properties: "builder_group:internal.client.r8"
         name: "rex"
       }
       mixins: "build_limited_scripts_slave recipe"