Add Finch testing config for PasswordBranding

testing/variations/fieldtrial_testing_config_ios.json

 {
     "AutofillFieldMetadata": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillProfileOrderByFrecency": [
         {
             "group_name": "EnabledLimitTo3",
             "params": {
@@ skipping 15 matching lines @@
         }
     ],
     "DataReductionProxyConfigService": [
         {
             "group_name": "Enabled",
             "params": {
                 "minimum_refresh_interval_on_success_msec": "21600000"
             }
         }
     ],
+    "PasswordBranding": [
+        {
+            "group_name": "SmartLockBranding"
+        }
+    ],
     "QUIC": [
         {
             "group_name": "Enabled",
             "params": {
                 "alternate_protocol_probability_threshold": "0.001"
             }
         }
     ],
     "SdchPersistence": [
         {
@@ skipping 15 matching lines @@
     ],
     "VarationsServiceControl": [
         {
             "group_name": "Interval_30min",
             "params": {
                 "fetch_period_min": "30"
             }
         }
     ]
 }