Media: Add Embed from URL tab.

Note: There is a bug that currently blocks adding several successive embeds.

see #21390.


git-svn-id: https://develop.svn.wordpress.org/trunk@22547 602fd350-edb4-49c9-b593-d223f7449a82

wp-admin/includes/meta-boxes.php

@@ -1053,7 +1053,7 @@ function post_thumbnail_meta_box( $post ) {
 			frame = wp.media( options );
 
 			frame.toolbar.on( 'activate:select', function() {
-				frame.toolbar.view().add({
+				frame.toolbar.view().set({
 					select: {
 						style: 'primary',
 						text:  update,