Media: Activate the featured image toolbar when the media dialog is first opened. see #21390, #21776.

git-svn-id: https://develop.svn.wordpress.org/trunk@22561 602fd350-edb4-49c9-b593-d223f7449a82

wp-admin/includes/meta-boxes.php

@@ -1084,6 +1084,8 @@ function post_thumbnail_meta_box( $post ) {
 					}
 				});
 			});
+
+			frame.toolbar.mode('select');
 		});
 
 		$element.on( 'click', '.remove', function( event ) {