Promote grandchildren of certain views are Views

Summary:
As an optimization, for something like a ScrollView which contains
a FlatViewGroup containing posts, make sure that each post is explicitly
mounted to a View. This may help improve performance, especially when said
Views are otherwise inlined as DrawCommands instead of actual Views.

Reviewed By: astreet

Differential Revision: D3161232

ReactAndroid/src/main/java/com/facebook/react/flat/FlatShadowNode.java

@@ -11,6 +11,7 @@ package com.facebook.react.flat;
 
 import javax.annotation.Nullable;
 
+import com.facebook.csslayout.CSSNode;
 import com.facebook.infer.annotation.Assertions;
 import com.facebook.react.uimanager.LayoutShadowNode;
 import com.facebook.react.uimanager.OnLayoutEvent;
@@ -55,6 +56,7 @@ import com.facebook.react.views.view.ReactClippingViewGroupHelper;
   private @Nullable DrawBackgroundColor mDrawBackground;
   private boolean mClipToBounds = false;
   private boolean mIsUpdated = true;
+  private boolean mForceMountChildrenToView;
   private float mClipLeft;
   private float mClipTop;
   private float mClipRight;
@@ -89,6 +91,20 @@ import com.facebook.react.views.view.ReactClippingViewGroupHelper;
     }
   }
 
+  /* package */ final void forceMountChildrenToView() {
+    if (mForceMountChildrenToView) {
+      return;
+    }
+
+    mForceMountChildrenToView = true;
+    for (int i = 0, childCount = getChildCount(); i != childCount; ++i) {
+      ReactShadowNode child = getChildAt(i);
+      if (child instanceof FlatShadowNode) {
+        ((FlatShadowNode) child).forceMountToView();
+      }
+    }
+  }
+
   /**
    * Collects DrawCommands produced by this FlatShadowNode.
    */
@@ -157,6 +173,14 @@ import com.facebook.react.views.view.ReactClippingViewGroupHelper;
     return mViewBottom - mViewTop;
   }
 
+  @Override
+  public void addChildAt(CSSNode child, int i) {
+    super.addChildAt(child, i);
+    if (mForceMountChildrenToView && child instanceof FlatShadowNode) {
+      ((FlatShadowNode) child).forceMountToView();
+    }
+  }
+
   /**
    * Marks root node as updated to trigger a StateBuilder pass to collect DrawCommands for the node
    * tree. Use it when FlatShadowNode is updated but doesn't require a layout pass (e.g. background