Decrease libfuzz mac luci bot's max concurrent builds to 1 from 3.

And add core count since it needs to only run on mac pros.

Bug: 790372
Change-Id: I41df0557fe05052bc0787553fb2cdd34142a7135
Reviewed-on: https://chromium-review.googlesource.com/c/1361298
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613660}

infra/config/global/cr-buildbucket.cfg

@@ -2018,11 +2018,7 @@ buckets {
     builders {
       name: "Libfuzzer Upload Mac ASan"
       dimensions: "os:Mac-10.13"
-      # Remove core count from the requested dimensions since the builder's
-      # pool has a mix of Mac models currently.
-      # TODO(crbug.com/790372): Remove this when the builder has a single Mac
-      # model in its pool.
-      dimensions: "cores:"
+      dimensions: "cores:24"
       mixins: "fyi-ci"
       # mixins: "fuzz-ci"
       mixins: "libfuzzer"

infra/config/global/luci-scheduler.cfg

@@ -3255,9 +3255,6 @@ job {
     bucket: "luci.chromium.ci"
     builder: "Libfuzzer Upload Mac ASan"
   }
-  triggering_policy: {
-    max_concurrent_invocations: 3
-  }
 }
 
 job {