jQuery: Replace use of the .load() method for the "load" event with .on( 'load', handler ).

The shortcut is deprecated since jQuery 1.8.

See http://api.jquery.com/load-event/.
See #35380.
Built from https://develop.svn.wordpress.org/trunk@36287


git-svn-id: http://core.svn.wordpress.org/trunk@36254 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/js/customize-controls.js

@@ -2870,7 +2870,7 @@
 
 				iframe = $( '<iframe />', { 'src': self.previewUrl(), 'title': api.l10n.previewIframeTitle } ).hide();
 				iframe.appendTo( self.container );
-				iframe.load( function() {
+				iframe.on( 'load', function() {
 					self.triedLogin = true;
 
 					iframe.remove();