Cron API: Add a new wp_doing_cron() helper function.

This replaces `DOING_CRON` checks via the constant.

Props tfrommen.
Fixes #39591.


git-svn-id: https://develop.svn.wordpress.org/trunk@40575 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/update.php

@@ -107,7 +107,7 @@ function get_core_checksums( $version, $locale ) {
 		$url = set_url_scheme( $url, 'https' );
 
 	$options = array(
-		'timeout' => ( ( defined('DOING_CRON') && DOING_CRON ) ? 30 : 3 ),
+		'timeout' => wp_doing_cron() ? 30 : 3,
 	);
 
 	$response = wp_remote_get( $url, $options );