Update config to add a new state to custom tab experiment

testing/variations/fieldtrial_testing_config_android.json

 {
     "EnhancedBookmarks": [
         {
             "group_name": "EnableEnhancedBookmarks",
             "params": {
                 "id": "Fake_id_to_trigger_experiment_code"
             }
         },
         {
             "group_name": "EnableEnhancedBookmarksGrid",
             "params": {
                 "DefaultViewMode": "Grid",
                 "id": "Fake_id_to_trigger_experiment_code"
             }
         },
         {
             "group_name": "EnableEnhancedBookmarksList",
             "params": {
                 "DefaultViewMode": "List",
                 "id": "Fake_id_to_trigger_experiment_code"
             }
         }
     ],
     "CustomTabs": [
         {
             "group_name": "Enabled"
         },
         {
+            "group_name": "DisablePrerender"
+        },
+        {
             "group_name": "Disabled"
         }
     ],
     "LimitSyncPassphrasePrompt": [
         {
             "group_name": "Enabled"
         }
     ],
     "ReaderModeUIFeedback": [
         {
             "group_name": "Show"
         },
         {
             "group_name": "DoNotShow"
         }
     ]
 }