It's used for "keyboard driven navigation" when setting up devices like Hangouts Meet. It lives in ash, but could probably be moved back to chrome. Unfortunately it has dependencies on the other accessibility "focus ring" layer code, which lives in ash.

See LoginDisplayHostWebUI:

if (system::InputDeviceSettings::Get()->ForceKeyboardDrivenUINavigation()) {
    views::FocusManager::set_arrow_key_traversal_enabled(true);
    //   crbug.com/405859  
    focus_ring_controller_ = std::make_unique<ash::FocusRingController>();
    focus_ring_controller_->SetVisible(true);

    keyboard_driven_oobe_key_handler_.reset(new KeyboardDrivenOobeKeyHandler);
  }