set leaf_nodes_builder in LayerTree::Flatten (flutter#34261)

Author: dnfield

flow/layers/layer_tree.cc

@@ -160,6 +160,7 @@ sk_sp<DisplayList> LayerTree::Flatten(const SkRect& bounds) {
       .frame_device_pixel_ratio      = device_pixel_ratio_,
       .layer_snapshot_store          = nullptr,
       .enable_leaf_layer_tracing     = false,
+      .leaf_nodes_builder            = builder.builder().get(),
       // clang-format on
   };
 

testing/dart/observatory/tracing_test.dart

@@ -65,6 +65,6 @@ void main() {
       }
     }
     expect(saveLayerRecordCount, 3);
-    expect(saveLayerCount, 6);
+    expect(saveLayerCount, 3);
   });
 }