[Extensions UNO] Add fieldtrial config

Bug: NONE
Change-Id: Ife54a9ff8d0d0cd80d04ab2f3f884c500ff6e236
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6309981
Commit-Queue: Kelvin Jiang <kelvinjiang@chromium.org>
Reviewed-by: Ryan Sultanem <rsult@google.com>
Auto-Submit: Kelvin Jiang <kelvinjiang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1426576}

testing/variations/fieldtrial_testing_config.json

@@ -9051,6 +9051,23 @@
             ]
         }
     ],
+    "EnableExtensionsExplicitBrowserSignin": [
+        {
+            "platforms": [
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "EnableExtensionsExplicitBrowserSignin"
+                    ]
+                }
+            ]
+        }
+    ],
     "EnableFallbackFontsCrashReporting": [
         {
             "platforms": [