Add field trial config for CSSExternalScanner experiment

testing/variations/fieldtrial_testing_config.json

 {
     "AndroidSessionNotifications": [
         {
             "platforms": [
                 "android"
             ],
             "experiments": [
                 {
                     "name": "Disabled"
                 }
@@ skipping 199 matching lines @@
                         "Background": "3;8;0.1;0;30000",
                         "BackgroundFileIO": "3;8;0.1;0;30000",
                         "Foreground": "8;32;0.3;0;30000",
                         "ForegroundFileIO": "8;32;0.3;0;30000",
                         "RedirectSequencedWorkerPools": "true"
                     }
                 }
             ]
         }
     ],
+    "CSSExternalScanner": [
+        {
+            "platforms": [
+                "win",
+                "mac",
+                "chromeos",
+                "linux",
+                "android"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled_ScanAndPreload",
+                    "params": {
+                        "cssExternalScannerNoPreload": "false",
+                        "cssExternalScannerPreload": "true"
+                    }
+                }
+            ]
+        }
+    ],
     "CaptivePortalInterstitial": [
         {
             "platforms": [
                 "chromeos",
                 "linux",
                 "mac",
                 "win"
             ],
             "experiments": [
                 {
@@ skipping 2089 matching lines @@
                 {
                     "name": "Enabled",
                     "enable_features": [
                         "use-new-media-cache"
                     ]
                 }
             ]
         }
     ]
 }