add experiment for running the JS on view creation

Summary:
Changelog: [Internal]

# Context

Whilst looking at Marketplace Loom traces, ShikaSD made the remark that we could potentially start the JS work much earlier instead of waiting for the View.onMeasure call, a behaviour that is already built-in Venice.

Reviewed By: ShikaSD

Differential Revision: D32559505

fbshipit-source-id: cc6337955ad2b6a6581a0347f1f976679eaca54d

ReactAndroid/src/main/java/com/facebook/react/ReactRootView.java

@@ -44,6 +44,7 @@ import com.facebook.react.bridge.UiThreadUtil;
 import com.facebook.react.bridge.WritableMap;
 import com.facebook.react.bridge.WritableNativeMap;
 import com.facebook.react.common.annotations.VisibleForTesting;
+import com.facebook.react.config.ReactFeatureFlags;
 import com.facebook.react.modules.appregistry.AppRegistry;
 import com.facebook.react.modules.core.DeviceEventManagerModule;
 import com.facebook.react.modules.deviceinfo.DeviceInfoModule;
@@ -439,7 +440,11 @@ public class ReactRootView extends FrameLayout implements RootView, ReactRoot {
       mInitialUITemplate = initialUITemplate;
 
       mReactInstanceManager.createReactContextInBackground();
-
+      // if in this experiment, we initialize the root earlier in startReactApplication
+      // instead of waiting for the initial measure
+      if (ReactFeatureFlags.enableEagerRootViewAttachment) {
+        attachToReactInstanceManager();
+      }
     } finally {
       Systrace.endSection(TRACE_TAG_REACT_JAVA_BRIDGE);
     }

ReactAndroid/src/main/java/com/facebook/react/config/ReactFeatureFlags.java

@@ -80,6 +80,9 @@ public class ReactFeatureFlags {
   /** Feature flag to configure eager initialization of MapBuffer So file */
   public static boolean enableEagerInitializeMapBufferSoFile = false;
 
+  /** Feature flag to configure eager attachment of the root view/initialisation of the JS code */
+  public static boolean enableEagerRootViewAttachment = false;
+
   private static boolean mapBufferSerializationEnabled = false;
 
   /** Enables or disables MapBuffer Serialization */