Make sure we're on the media grid page before calling wp.media().

props azaozz.
fixes #29520 for trunk.

git-svn-id: https://develop.svn.wordpress.org/trunk@29725 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/upload.php

@@ -57,7 +57,7 @@ if ( 'grid' === $mode ) {
 
 	require_once( ABSPATH . 'wp-admin/admin-header.php' );
 	?>
-	<div class="wrap">
+	<div class="wrap" id="wp-media-grid">
 		<h2>
 		<?php
 		echo esc_html( $title );