Suppress ie_chalkboard for perf cq on MacOS

Marked ie_chalkboard story as experimental on MacOS because of recent flakiness of test.

Bug: 1133599
Change-Id: If93c0594ad79259042b6e574d6ac59ce034f2049
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2456306
Auto-Submit: Behdad Bakhshinategh <behdadb@chromium.org>
Commit-Queue: Behdad Bakhshinategh <behdadb@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814693}

testing/scripts/representative_perf_test_data/representatives_frame_times_upper_limit.json

@@ -133,7 +133,9 @@
     "ie_chalkboard": {
       "ci_095": 32.615,
       "avg": 62.871,
-      "cpu_wall_time_ratio": 0.491
+      "cpu_wall_time_ratio": 0.491,
+      "experimental": true,
+      "_comment": "crbug.com/1133599"
     },
     "new_tilings": {
       "ci_095": 0.146,