Customize: Add wp-embed-responsive class to body if using widgets block editor

Appends the wp-embed-responsive class to the body if the widgets block editor is
enabled. This fixes a bug preventing embeds (e.g. YouTube embeds) appearing in
the widgets block editor.

Fixes #53609.
Props walbo, kevin940726.

Built from https://develop.svn.wordpress.org/trunk@51716


git-svn-id: http://core.svn.wordpress.org/trunk@51322 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/customize.php

@@ -144,6 +144,10 @@ if ( is_rtl() ) {
 }
 $body_class .= ' locale-' . sanitize_html_class( strtolower( str_replace( '_', '-', get_user_locale() ) ) );
 
+if ( wp_use_widgets_block_editor() ) {
+	$body_class .= ' wp-embed-responsive';
+}
+
 $admin_title = sprintf( $wp_customize->get_document_title_template(), __( 'Loading…' ) );
 
 ?>