Default enable main frame before activation and remove finch experiment.

testing/variations/fieldtrial_testing_config_mac.json

 {
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillFieldMetadata": [
         {
             "group_name": "Enabled"
         }
@@ skipping 116 matching lines @@
     "IntelligentSessionRestore": [
         {
             "group_name": "Enabled"
         }
     ],
     "MacMemoryMechanism": [
         {
             "group_name": "Mach"
         }
     ],
-    "MainFrameBeforeActivation": [
-        {
-            "enable_features": [
-                "MainFrameBeforeActivation"
-            ],
-            "group_name": "Enabled"
-        }
-    ],
     "MaterialDesignDownloads": [
         {
             "group_name": "Enabled"
         }
     ],
     "MojoChannel": [
         {
             "group_name": "Enabled"
         }
     ],
@@ skipping 280 matching lines @@
     ],
     "use-new-media-cache": [
         {
             "enable_features": [
                 "use-new-media-cache"
             ],
             "group_name": "Enabled"
         }
     ]
 }