Add json config for field group that fetches warmup URL

testing/variations/fieldtrial_testing_config.json

 {
     "AlternateComponentUrls": [
         {
             "platforms": [
                 "android",
                 "chromeos",
                 "ios",
                 "linux",
                 "mac",
                 "win"
@@ skipping 455 matching lines @@
         {
             "platforms": [
                 "android",
                 "chromeos",
                 "linux",
                 "mac",
                 "win"
             ],
             "experiments": [
                 {
-                    "name": "Enabled"
+                    "name": "Enabled_Warmup",
+                    "params": {
+                        "enable_warmup": "true"
+                    }
                 }
             ]
         }
     ],
     "DefaultBrowserInfobar": [
         {
             "platforms": [
                 "win"
             ],
             "experiments": [
@@ skipping 2348 matching lines @@
                 {
                     "name": "Enabled",
                     "enable_features": [
                         "use-new-media-cache"
                     ]
                 }
             ]
         }
     ]
 }