Reland of Default enable main frame before activation and remove finch experiment.

testing/variations/fieldtrial_testing_config_android.json

 {
     "AllBookmarks": [
         {
             "disable_features": [
                 "AllBookmarks"
             ],
             "group_name": "Disabled"
         }
     ],
     "AndroidSessionNotifications": [
@@ skipping 180 matching lines @@
         }
     ],
     "LowPriorityIFrames": [
         {
             "group_name": "Enabled",
             "params": {
                 "lowPriorityIframes": "true"
             }
         }
     ],
-    "MainFrameBeforeActivation": [
-        {
-            "enable_features": [
-                "MainFrameBeforeActivation"
-            ],
-            "group_name": "Enabled"
-        }
-    ],
     "ModerateBindingOnBackgroundTabCreation": [
         {
             "group_name": "Enabled"
         }
     ],
     "MojoChannel": [
         {
             "group_name": "Enabled"
         }
     ],
@@ skipping 399 matching lines @@
     ],
     "use-new-media-cache": [
         {
             "enable_features": [
                 "use-new-media-cache"
             ],
             "group_name": "Enabled"
         }
     ]
 }