Add Precache global ranking config to test list.

testing/variations/fieldtrial_testing_config.json

 {
     "AndroidSessionNotifications": [
         {
             "platforms": [
                 "android"
             ],
             "experiments": [
                 {
                     "name": "Disabled"
                 }
@@ skipping 1554 matching lines @@
             ]
         }
     ],
     "Precache": [
         {
             "platforms": [
                 "android"
             ],
             "experiments": [
                 {
-                    "name": "Enabled"
+                    "name": "EnabledGlobalMedium",
+                    "params": {
+                        "config_url": "https://www.gstatic.com/chrome/wifiprefetch/precache_config_gm"
+                    }
                 }
             ]
         }
     ],
     "PreconnectMore": [
         {
             "platforms": [
                 "android",
                 "chromeos",
                 "linux",
@@ skipping 949 matching lines @@
                 {
                     "name": "Enabled",
                     "enable_features": [
                         "use-new-media-cache"
                     ]
                 }
             ]
         }
     ]
 }