Reland "Enable WebUITabStrip in fieldtrial_testing_config.json"

Change was reverted due to benign TSan failures. They should be
gone as of https://crrev.com/c/2076278

This is a reland of 756673ee

Original change's description:
> Enable WebUITabStrip in fieldtrial_testing_config.json
>
> Bug: 1009881
> Change-Id: I0c784e4ac16bb14e5515b95680adfea3b5e3148c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2069499
> Reviewed-by: Jesse Doherty <jwd@chromium.org>
> Commit-Queue: Collin Baker <collinbaker@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#744385}

TBR=jwd@chromium.org

Bug: 1009881
Change-Id: I9bcb92aa29bcbf8d4d03e54b00b80d3b7b3826b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2079646Reviewed-by: Collin Baker <collinbaker@chromium.org>
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745675}

testing/variations/fieldtrial_testing_config.json

@@ -6503,6 +6503,23 @@
             ]
         }
     ],
+    "WebUITabStrip": [
+        {
+            "platforms": [
+                "chromeos",
+                "linux",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "WebUITabStrip"
+                    ]
+                }
+            ]
+        }
+    ],
     "WebViewWakeMetricsService": [
         {
             "platforms": [