Add AppContainer mitigation to finch testing parameters.

testing/variations/fieldtrial_testing_config_win.json

 {
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillFieldMetadata": [
         {
             "group_name": "Enabled"
         }
@@ skipping 62 matching lines @@
         {
             "disable_features": [
                 "DocumentWriteEvaluator"
             ],
             "group_name": "DocumentWriteScriptBlockGroup",
             "params": {
                 "disallowFetchForDocWrittenScriptsInMainFrameOnSlowConnections": "true"
             }
         }
     ],
+    "EnableAppContainer": [
+        {
+            "group_name": "Enabled"
+        }
+    ],
     "EnableGoogleCachedCopyTextExperiment": [
         {
             "group_name": "Button"
         }
     ],
     "EnableMediaRouter": [
         {
             "group_name": "Enabled"
         }
     ],
@@ skipping 450 matching lines @@
     ],
     "use-new-media-cache": [
         {
             "enable_features": [
                 "use-new-media-cache"
             ],
             "group_name": "Enabled"
         }
     ]
 }