Flagged css_value_type_shadow as experimental to prevent flakes

css_value_type_shadow has been disabled on representative perf tests,
because of increased number of flakes.

Bug: chromium:1093313
Change-Id: Ic99f9c06b0e996b1a747b65079173e7daec3c0ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2239672
Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Behdad Bakhshinategh <behdadb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#776993}

testing/scripts/representative_perf_test_data/representatives_frame_times_upper_limit.json

@@ -57,7 +57,9 @@
     },
     "css_value_type_shadow": {
       "ci_095": 1.284,
-      "avg": 49.386
+      "avg": 49.386,
+      "experimental": true,
+      "_comment": "crbug.com/1093313"
     },
     "nvidia_vertex_buffer_object": {
       "ci_095": 3.37,
@@ -87,7 +89,9 @@
     },
     "css_value_type_shadow": {
       "ci_095": 13.611,
-      "avg": 61.261
+      "avg": 61.261,
+      "experimental": true,
+      "_comment": "crbug.com/1093313"
     },
     "animometer_webgl_attrib_arrays": {
       "ci_095": 0.495,