Add field trial testing config for MojoChannel

testing/variations/fieldtrial_testing_config_android.json

 {
     "AndroidSessionNotifications": [
         {
             "group_name": "Disabled"
         }
     ],
     "AsyncDns": [
         {
             "group_name": "AsyncDnsA"
         },
@@ skipping 142 matching lines @@
             "params": {
                 "Enabled": "true"
             }
         }
     ],
     "ModerateBindingOnBackgroundTabCreation": [
         {
             "group_name": "Enabled"
         }
     ],
+    "MojoChannel": [
+        {
+            "group_name": "Enabled"
+        }
+    ],
     "NTPPopularSites": [
         {
             "group_name": "Enabled5",
             "params": {
                 "version": "5"
             }
         }
     ],
     "NetworkQualityEstimator": [
         {
@@ skipping 297 matching lines @@
     ],
     "use-new-media-cache": [
         {
             "enable_features": [
                 "use-new-media-cache"
             ],
             "group_name": "Enabled"
         }
     ]
 }