[Extensions Toolbar] Add a finch config for the redesign

testing/variations/fieldtrial_testing_config_linux.json

 {
     "AutofillFieldMetadata": [
         {
             "group_name": "Enabled"
         }
     ],
     "CaptivePortalInterstitial": [
         {
             "group_name": "Enabled"
         }
@@ skipping 11 matching lines @@
     "EnableGoogleCachedCopyTextExperiment": [
         {
             "group_name": "Button"
         }
     ],
     "EnableSessionCrashedBubbleUI": [
         {
             "group_name": "Enabled"
         }
     ],
+    "ExtensionActionRedesign": [
+        {
+            "group_name": "Enabled"
+        }
+    ],
     "InstanceID": [
         {
             "group_name": "Enabled"
         }
     ],
     "PluginPowerSaver": [
         {
             "group_name": "Enabled"
         }
     ],
@@ skipping 75 matching lines @@
     ],
     "VarationsServiceControl": [
         {
             "group_name": "Interval_30min",
             "params": {
                 "fetch_period_min": "30"
             }
         }
     ]
 }