WordPress at a glance
function is not described

wp_login_viewport_meta() WP 3.7.0

No Hooks.

Return

Nothing (null).

Usage

wp_login_viewport_meta();

Code of wp login viewport meta: wp-login.php VER 5.0.3

<?php
function wp_login_viewport_meta() {
	?>
	<meta name="viewport" content="width=device-width" />
	<?php
}