Constructs an OverlaySubscriber object.
\Drupal\Core\ContentNegotiation $negotiation: The content negotiation service.
\Drupal\user\UserData $user_data: The user.data service.
public function __construct(ContentNegotiation $negotiation, UserData $user_data) {
$this->negotiation = $negotiation;
$this->userData = $user_data;
}