Add Finch testing config for PasswordBranding

testing/variations/fieldtrial_testing_config_mac.json

 {
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillFieldMetadata": [
         {
             "group_name": "Enabled"
         }
@@ skipping 57 matching lines @@
     "IntelligentSessionRestore": [
         {
             "group_name": "Enabled"
         }
     ],
     "MaterialDesignDownloads": [
         {
             "group_name": "Enabled"
         }
     ],
+    "PasswordBranding": [
+        {
+            "group_name": "SmartLockBranding"
+        }
+    ],
     "QUIC": [
         {
             "group_name": "Enabled",
             "params": {
                 "alternate_protocol_probability_threshold": "0.01"
             }
         }
     ],
     "RefreshTokenDeviceId": [
         {
@@ skipping 96 matching lines @@
         {
             "group_name": "Enabled"
         }
     ],
     "WebRTC-PeerConnectionDTLS1.2": [
         {
             "group_name": "Enabled"
         }
     ]
 }