Back compat $_REQUEST['post_id'] for media XHR requests.

Props koopersmith, nacin.
fixes #22588


git-svn-id: https://develop.svn.wordpress.org/trunk@22865 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/script-loader.php

@@ -324,6 +324,7 @@ function wp_default_scripts( &$scripts ) {
 		'settings' => array(
 			'saveAttachmentNonce' => wp_create_nonce( 'save-attachment' ),
 			'ajaxurl' => admin_url( 'admin-ajax.php', 'relative' ),
+			'postId'  => 0,
 		),
 	) );