Suppress flaky story

CSS_value_type_shadow story has been flaky on mac (representative perf tests)
To prevent flakiness the story is tagged as experimental

Bug: 1139155
Change-Id: If8329af22bb3b8b11b768ecc5620d8ba0919c5cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2485782
Auto-Submit: Behdad Bakhshinategh <behdadb@chromium.org>
Commit-Queue: Behdad Bakhshinategh <behdadb@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Reviewed-by: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819002}

testing/scripts/representative_perf_test_data/representatives_frame_times_upper_limit.json

@@ -112,7 +112,9 @@
     "css_value_type_shadow": {
       "ci_095": 18.3,
       "avg": 65.502,
-      "cpu_wall_time_ratio": 0.585
+      "cpu_wall_time_ratio": 0.585,
+      "experimental": true,
+      "_comment": "crbug.com/1139155"
     },
     "animometer_webgl_attrib_arrays": {
       "ci_095": 0.44,