Enable main frame before activation on waterfall.

testing/variations/fieldtrial_testing_config_android.json

 {
     "AndroidSessionNotifications": [
         {
             "group_name": "Disabled"
         }
     ],
     "AsyncDns": [
         {
             "group_name": "AsyncDnsA"
         },
@@ skipping 135 matching lines @@
         }
     ],
     "LowPriorityIFrames": [
         {
             "group_name": "Enabled",
             "params": {
                 "lowPriorityIframes": "true"
             }
         }
     ],
+    "MainFrameBeforeActivation": [
+        {
+            "enable_features": [
+                "MainFrameBeforeActivation"
+            ],
+            "group_name": "Enabled"
+        }
+    ],
     "MinidumpDirectoryObserver": [
         {
             "group_name": "ObserverExperimentGroup",
             "params": {
                 "Enabled": "true"
             }
         }
     ],
     "ModerateBindingOnBackgroundTabCreation": [
         {
@@ skipping 355 matching lines @@
     ],
     "use-new-media-cache": [
         {
             "enable_features": [
                 "use-new-media-cache"
             ],
             "group_name": "Enabled"
         }
     ]
 }