Use Mac for Mac builder

Change-Id: Idd2c161072a1b2e26df41433badd7b8215e2b75b
diff --git a/cr-buildbucket.cfg b/cr-buildbucket.cfg
index 70f0d21..969717f 100644
--- a/cr-buildbucket.cfg
+++ b/cr-buildbucket.cfg
@@ -52,7 +52,7 @@
     builders {
       category: "Jiri"
       name: "Mac x86-64"
-      dimensions: "os:Ubuntu-14.04"
+      dimensions: "os:Mac-10.12"
       dimensions: "cpu:x86-64"
       recipe {
         name: "jiri"