Add field trial config for CSSExternalScanner experiment

testing/variations/fieldtrial_testing_config.json

Index: testing/variations/fieldtrial_testing_config.json
diff --git a/testing/variations/fieldtrial_testing_config.json b/testing/variations/fieldtrial_testing_config.json
index 0e17cfcbb73ee9b1247e528ed7c1a7a8283388b4..bab57c04ea767f5c7e783623b78e4798b4b63ff4 100644
--- a/testing/variations/fieldtrial_testing_config.json
+++ b/testing/variations/fieldtrial_testing_config.json
@@ -217,6 +217,26 @@
             ]
         }
     ],
+    "CSSExternalScanner": [
+        {
+            "platforms": [
+                "win",
+                "mac",
+                "chromeos",
+                "linux",
+                "android"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled_ScanAndPreload",
+                    "params": {
+                        "cssExternalScannerNoPreload": "false",
+                        "cssExternalScannerPreload": "true"
+                    }
+                }
+            ]
+        }
+    ],
     "CaptivePortalInterstitial": [
         {
             "platforms": [