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/class-plugin-upgrader.php

@@ -406,7 +406,7 @@ class Plugin_Upgrader extends WP_Upgrader {
 			return $return;
 
 		// When in cron (background updates) don't deactivate the plugin, as we require a browser to reactivate it
-		if ( defined( 'DOING_CRON' ) && DOING_CRON )
+		if ( wp_doing_cron() )
 			return $return;
 
 		$plugin = isset($plugin['plugin']) ? $plugin['plugin'] : '';