Enabling field trial testing for AsyncSetAsDefault

testing/variations/fieldtrial_testing_config_win.json

 {
+    "AsyncSetAsDefault": [
+        {
+            "group_name": "Enabled"
+        }
+    ],
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillFieldMetadata": [
         {
             "group_name": "Enabled"
         }
     ],
@@ skipping 214 matching lines @@
                 "fetch_period_min": "30"
             }
         }
     ],
     "WebRTC-PeerConnectionDTLS1.2": [
         {
             "group_name": "Enabled"
         }
     ]
 }