Customizer: Replace usage of calc() with box-sizing: border-box.

IE8 doesn't support `calc()`.

props afercia.
see #31336.
fixes #32664.
Built from https://develop.svn.wordpress.org/trunk@32821


git-svn-id: http://core.svn.wordpress.org/trunk@32792 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/customize-nav-menus.css

@@ -31,7 +31,7 @@
 }
 
 .wp-customizer .menu-item-bar .menu-item-handle {
-	max-width: 100%;
+	width: 100%;
 	background: #fff;
 }