Fix mac jumbo try builder core dimension

Bug: 782863
Change-Id: I9565b84c027f0c88dfb4ee02602a2d49c3a95016
Reviewed-on: https://chromium-review.googlesource.com/1029218Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553881}

infra/config/global/cr-buildbucket.cfg

@@ -1266,7 +1266,11 @@ buckets {
     builders { mixins: "mac-angle-try" name: "mac_angle_compile_dbg_ng" }
     builders { mixins: "mac-angle-try" name: "mac_angle_dbg_ng" }
     builders { mixins: "mac-angle-try" name: "mac_angle_rel_ng" }
-    builders { mixins: "mac-try" name: "mac-jumbo-rel" }
+    builders {
+      mixins: "mac-try"
+      name: "mac-jumbo-rel"
+      dimensions: "cores:4"
+    }
     # NOTE: the folliwing 3 trybots aren't sensitive to Mac version on which
     # they are built, hence no additional dimension is specified.
     # The 10.xx version translates to which bots will run isolated tests.