Coverage: Change the cores of mac-code-coverage-generation from 8 to 24

I just realized luci.chromium.ci sets the default value of cores to 8,
however, the mac-code-coverage-generation builder has 24 cores:
https://chromium-swarm.appspot.com/bot?id=build227-m9

Bug: 825362
Change-Id: If017fca1c47b798c2573f50d37e1a7823595181c
Reviewed-on: https://chromium-review.googlesource.com/1098645Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567084}

infra/config/global/cr-buildbucket.cfg

@@ -1394,6 +1394,7 @@ buckets {
     builders {
       name: "mac-code-coverage-generation"
       mixins: "code-coverage"
+      dimensions: "cores:24"
     }
     ############################################################################
     # This Section of luci.chromium.ci is managed by luci-check #