[cr-buildbucket.cfg] Make mac-try use cores:

Since they could be 4 core or 8 core.

Bug: 840631
TBR: tandrii
Change-Id: I23e40e9912116f06905b6f31eff74548c5a5afda
Reviewed-on: https://chromium-review.googlesource.com/1109017Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Ryan Tseng <hinoka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569071}

infra/config/global/cr-buildbucket.cfg

@@ -352,6 +352,8 @@ builder_mixins {
 builder_mixins {
   name: "mac-try"
   mixins: "mac"
+  # mac bots may run both on Mac Minis with cores:4 and VMs with cores:8.
+  dimensions: "cores:"
   recipe {
     properties: "mastername:tryserver.chromium.mac"
     properties_j: "$depot_tools/bot_update:{\"apply_patch_on_gclient\":true}"
@@ -399,8 +401,6 @@ builder_mixins {
   name: "ios-try"
   mixins: "mac-try"
   mixins: "xcode-caches"
-  # ios bots may run both on Mac Minis with cores:4 and VMs with cores:8.
-  dimensions: "cores:"
   recipe {
     name: "ios/try"
   }