  static SpellCheckerHolder* FromRenderFrame(
      content::RenderFrame* render_frame) {
    for (auto* holder : instances_) {
      if (holder->render_frame() == render_frame)
        return holder;
    }
    return nullptr;
  }
  static SpellCheckerHolder* FromRenderFrame(
      content::RenderFrame* render_frame) {
    for (auto* holder : instances_) {
      if (holder->render_frame() == render_frame)
        return holder;
    }
    return nullptr;
  }