Implement a skeleton of the Superfish interstitial

testing/variations/fieldtrial_testing_config.json

 {
     "AlternateComponentUrls": [
         {
             "platforms": [
                 "android",
                 "chromeos",
                 "ios",
                 "linux",
                 "mac",
                 "win"
@@ skipping 2972 matching lines @@
                         "SubresourceFilter"
                     ]
                 },
                 {
                     "name": "VerifyFeatureIsStillDisabledByDefault",
                     "forcing_flag": "suppress-enabling-subresource-filter-from-fieldtrial-testing-config"
                 }
             ]
         }
     ],
+    "SuperfishInterstitial": [
+        {
+            "platforms": [
+                "win"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "SuperfishInterstitial"
+                    ]
+                }
+            ]
+        }
+    ],
     "SyncUSSAutocomplete": [
         {
             "platforms": [
                 "android",
                 "chromeos",
                 "ios",
                 "linux",
                 "mac",
                 "win"
             ],
@@ skipping 427 matching lines @@
                 {
                     "name": "Enabled",
                     "enable_features": [
                         "use-new-media-cache"
                     ]
                 }
             ]
         }
     ]
 }