Enable main frame before activation on waterfall.

testing/variations/fieldtrial_testing_config_mac.json

 {
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillFieldMetadata": [
         {
             "group_name": "Enabled"
         }
@@ skipping 108 matching lines @@
     "LocalNTPSuggestionsService": [
         {
             "group_name": "Enabled"
         }
     ],
     "MacMemoryMechanism": [
         {
             "group_name": "Mach"
         }
     ],
+    "MainFrameBeforeActivation": [
+        {
+            "enable_features": [
+                "MainFrameBeforeActivation"
+            ],
+            "group_name": "Enabled"
+        }
+    ],
     "MaterialDesignDownloads": [
         {
             "group_name": "Enabled"
         }
     ],
     "MojoChannel": [
         {
             "group_name": "Enabled"
         }
     ],
@@ skipping 268 matching lines @@
     ],
     "use-new-media-cache": [
         {
             "enable_features": [
                 "use-new-media-cache"
             ],
             "group_name": "Enabled"
         }
     ]
 }