Remove enableDrawMutationFix feature flag and ship it

Summary:
This was added to prevent mutating the UI during draw or measure, and appears to have been effective. Keep the comments, ship the feature, remove flags.

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D25258409

fbshipit-source-id: 36ad8a03d1eb82bc9dcd769372c03f1ebe8b8da8

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

@@ -86,9 +86,6 @@ public class ReactFeatureFlags {
   /** Disable customDrawOrder in ReactViewGroup under Fabric only. */
   public static boolean disableCustomDrawOrderFabric = false;
 
-  /** Potential bugfix for crashes caused by mutating the view hierarchy during onDraw. */
-  public static boolean enableDrawMutationFix = true;
-
   /** Use lock-free data structures for Fabric MountItems. */
   public static boolean enableLockFreeMountInstructions = false;
 }

ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java

@@ -586,11 +586,11 @@ public void synchronouslyUpdateViewOnUIThread(
 
     int commitNumber = mCurrentSynchronousCommitNumber++;
 
-    // We are on the UI thread so this would otherwise be safe to call, *BUT* we don't know
-    // where we are on the callstack. Why isn't this safe, and why do we have additional safeguards
-    // here?
+    // We are on the UI thread so it would otherwise be safe to call `tryDispatchMountItems` here to
+    // flush previously-queued mountitems, *BUT* we don't know where we are on the callstack.
+    // Why isn't it safe, and why do we have additional safeguards here?
     //
-    // A tangible example where this will cause a crash:
+    // A tangible example where it would cause a crash, and did in the past:
     // 1. There are queued "delete" mutations
     // 2. We're called by this stack trace:
     //    FabricUIManager.synchronouslyUpdateViewOnUIThread(FabricUIManager.java:574)
@@ -607,9 +607,7 @@ public void synchronouslyUpdateViewOnUIThread(
     //    android.widget.ScrollView.overScrollBy(ScrollView.java:2040)
     //    android.widget.ScrollView.computeScroll(ScrollView.java:1481)
     //    android.view.View.updateDisplayListIfDirty(View.java:20466)
-    if (!ReactFeatureFlags.enableDrawMutationFix) {
-      tryDispatchMountItems();
-    }
+    // 3. A view is deleted while its parent is being drawn, causing a crash.
 
     MountItem synchronousMountItem =
         new MountItem() {