Add OffMainThreadServiceWorkerStartup to fieldtrial_testing_config.json

Bug: 692909
Change-Id: If4c656288728f6c52440fc4b2e834e3e2b6e8c61
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1846611Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704454}

testing/variations/fieldtrial_testing_config.json

@@ -3767,6 +3767,26 @@
             ]
         }
     ],
+    "OffMainThreadServiceWorkerStartup": [
+        {
+            "platforms": [
+                "android",
+                "android_webview",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "OffMainThreadServiceWorkerStartup"
+                    ]
+                }
+            ]
+        }
+    ],
     "OfferUploadCreditCards": [
         {
             "platforms": [