message: Reserve space in a vector before adding elements.
diff: @@ -60,6 +60,8 @@ void AdoptLayoutAndFlattenIfSameType(const LayoutTree& source,
  const auto& first_subitem = source.Children().front().Value();
  CHECK(src_item.MustWrap() == first_subitem.MustWrap());
  CHECK(src_item.SpacesBefore() == first_subitem.SpacesBefore());
  + destination->Children().reserve(destination->Children().size() +
  + source.Children().size());
  for (const auto& sublayout : source.Children())
  destination->AdoptSubtree(sublayout);
  } else {
  