Customizer: Avoid SecurityErrors when the Customizer is embedded in an origin other than wp-admin.

props mattwiebe.
fixes #31687.

git-svn-id: https://develop.svn.wordpress.org/trunk@31885 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/js/customize-loader.js

@@ -155,6 +155,10 @@ window.wp = window.wp || {};
 				Loader.saved( false );
 			} );
 
+			this.messenger.bind( 'title', function( newTitle ){
+				window.document.title = newTitle;
+			});
+
 			this.pushState( src );
 
 			this.trigger( 'open' );