AJAX: add a new function, wp_doing_ajax(), which can replace... (wait for it...) DOING_AJAX checks via the constant.

Props Mte90, sebastian.pisula, swissspidy.
Fixes #25669.


git-svn-id: https://develop.svn.wordpress.org/trunk@38334 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-customize-manager.php

@@ -315,8 +315,7 @@ final class WP_Customize_Manager {
 	 * @return bool True if it's an Ajax request, false otherwise.
 	 */
 	public function doing_ajax( $action = null ) {
-		$doing_ajax = ( defined( 'DOING_AJAX' ) && DOING_AJAX );
-		if ( ! $doing_ajax ) {
+		if ( ! wp_doing_ajax() ) {
 			return false;
 		}