Modify testing group name for AutomaticTabDiscarding.

testing/variations/fieldtrial_testing_config_mac.json

 {
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillFieldMetadata": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillProfileOrderByFrecency": [
         {
             "group_name": "EnabledLimitTo3",
             "params": {
                 "limit": "3"
             }
         }
     ],
     "AutomaticTabDiscarding": [
         {
             "enable_features": [
                 "AutomaticTabDiscarding"
             ],
-            "group_name": "Enabled_Once"
+            "group_name": "Enabled_Once_5"
         }
     ],
     "CaptivePortalInterstitial": [
         {
             "group_name": "Enabled"
         }
     ],
     "ChromeDashboard": [
         {
             "group_name": "Enabled"
@@ skipping 175 matching lines @@
         {
             "group_name": "Enabled"
         }
     ],
     "WebRTC-PeerConnectionDTLS1.2": [
         {
             "group_name": "Enabled"
         }
     ]
 }