testing config: Enable ServiceWorkerOnUI.

For increased test coverage. I plan to start enabling it on
Dev/Canary soon once existing bugs are addressed, and eventually
launch to Stable.

Bug: 824858
Change-Id: I71374bdc94b32ba24fcbbd8ab86df90c735dd637
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786881
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695883}

testing/variations/fieldtrial_testing_config.json

@@ -5261,6 +5261,24 @@
             ]
         }
     ],
+    "ServiceWorkerOnUI": [
+        {
+            "platforms": [
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "ServiceWorkerOnUI"
+                    ]
+                }
+            ]
+        }
+    ],
     "SettingsEnforcement": [
         {
             "platforms": [