Enable WebFonts intervention v2 for slow2g on variation test waterfall

testing/variations/fieldtrial_testing_config_win.json

 {
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillFieldMetadata": [
         {
             "group_name": "Enabled"
         }
@@ skipping 450 matching lines @@
         }
     ],
     "VarationsServiceControl": [
         {
             "group_name": "Interval_30min",
             "params": {
                 "fetch_period_min": "30"
             }
         }
     ],
+    "WebFontsInterventionV2": [
+        {
+            "group_name": "Enabled-slow2g"
+        }
+    ],
     "WebRTC-EnableWebRtcEcdsa": [
         {
             "enable_features": [
                 "WebRTC-EnableWebRtcEcdsa"
             ],
             "group_name": "Enabled"
         }
     ],
     "WebRTC-H264WithOpenH264FFmpeg": [
         {
@@ skipping 15 matching lines @@
     ],
     "use-new-media-cache": [
         {
             "enable_features": [
                 "use-new-media-cache"
             ],
             "group_name": "Enabled"
         }
     ]
 }