Add field trial testing config for MojoChannel

testing/variations/fieldtrial_testing_config_win.json

 {
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillFieldMetadata": [
         {
             "group_name": "Enabled"
         }
@@ skipping 119 matching lines @@
     "LocalNTPSuggestionsService": [
         {
             "group_name": "Enabled"
         }
     ],
     "MaterialDesignDownloads": [
         {
             "group_name": "Enabled"
         }
     ],
+    "MojoChannel": [
+        {
+            "group_name": "Enabled"
+        }
+    ],
     "NetworkQualityEstimator": [
         {
             "group_name": "Enabled",
             "params": {
                 "2G.DefaultMedianKbps": "81",
                 "2G.DefaultMedianRTTMsec": "1337",
                 "3G.DefaultMedianKbps": "658",
                 "3G.DefaultMedianRTTMsec": "297",
                 "4G.DefaultMedianKbps": "1756",
                 "4G.DefaultMedianRTTMsec": "159",
@@ skipping 249 matching lines @@
     ],
     "use-new-media-cache": [
         {
             "enable_features": [
                 "use-new-media-cache"
             ],
             "group_name": "Enabled"
         }
     ]
 }