Adding a previews blacklist transtion entry to fieldtrial list

testing/variations/fieldtrial_testing_config.json

 {
     "AlternateComponentUrls": [
         {
             "platforms": [
                 "android",
                 "chromeos",
                 "ios",
                 "linux",
                 "mac",
                 "win"
@@ skipping 604 matching lines @@
                     "params": {
                         "persistent_menu_item_enabled": "true"
                     },
                     "enable_features": [
                         "DataReductionProxyMainMenu"
                     ]
                 }
             ]
         }
     ],
+    "DataReductionProxyPreviewsBlacklistTransition": [
+        {
+            "platforms": [
+                "android"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled"
+                }
+            ]
+        }
+    ],
     "DataReductionProxyServerExperiments": [
         {
             "platforms": [
                 "android"
             ],
             "experiments": [
                 {
                     "name": "Brotli",
                     "params": {
                         "exp": "allow_brotli"
@@ skipping 2814 matching lines @@
                 {
                     "name": "Enabled",
                     "enable_features": [
                         "use-new-media-cache"
                     ]
                 }
             ]
         }
     ]
 }