Mark aquarium_20K story as experimental on perf cq [win]

The flaky story on perf cq windows is marked as experimental to prevent further flakiness.

Bug: 1134251
Change-Id: Ie99b7b0d62b22c86105a53fbca74d0eeb322b514
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462392
Commit-Queue: Behdad Bakhshinategh <behdadb@chromium.org>
Auto-Submit: Behdad Bakhshinategh <behdadb@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815767}

testing/scripts/representative_perf_test_data/representatives_frame_times_upper_limit.json

@@ -33,7 +33,9 @@
     "aquarium_20k": {
       "ci_095": 45.797,
       "avg": 139.949,
-      "cpu_wall_time_ratio": 0.455
+      "cpu_wall_time_ratio": 0.455,
+      "experimental": true,
+      "_comment": "crbug.com/1134251"
     },
     "balls_css_transition_2_properties": {
       "ci_095": 7.166,