[mac-rel-swarming] Run content_browsertests only on 8+ cores instances

The content_browsertests is taking 35 mins on n1-highcpu-8 instances, and would timeout on 4 cores instances.
Because there are only 3 instances in the mac-rel-swarming pool, increasing shard might cause other tests starving, this CL is limiting the content_browsertests to use 8+ cores instances only.

Bug: 1255940
Change-Id: Ibee3185a9b144771b52f78ac0118ad1a5ed88b87
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3218132
Auto-Submit: Kuan Huang <[email protected]>
Commit-Queue: Takuto Ikuta <[email protected]>
Reviewed-by: Takuto Ikuta <[email protected]>
Cr-Commit-Position: refs/heads/main@{#930386}
diff --git a/testing/buildbot/test_suite_exceptions.pyl b/testing/buildbot/test_suite_exceptions.pyl
index 01728ec..a53e748 100644
--- a/testing/buildbot/test_suite_exceptions.pyl
+++ b/testing/buildbot/test_suite_exceptions.pyl
@@ -1340,6 +1340,17 @@
           '--test-launcher-filter-file=../../testing/buildbot/filters/linux-lacros.content_browsertests.filter',
         ],
       },
+      # https://crbug.com/1255940
+      'mac-rel-swarming': {
+        'swarming': {
+          'dimension_sets': [
+            {
+              # These test would time out when running on 4 cores instances.
+              'cores': '8|12',
+            }
+          ],
+        },
+      },
     },
   },
   'content_browsertests_wayland': {