Script Loader: Use dns-prefetch for the Emoji CDN.

* `preconnect` will be potentially pretty heavy on the CDN. With the Unicode 9.0 emoji update, almost all browsers will trigger the `preconnect`.
* `preconnect` only opens one connection, but `s.w.org` is HTTP/1.1, so the browser will use the preconnected connection for the first emoji, then it has to open new connections for subsequent emoji.

Also use the same URL as we use for the `emoji_svg_url` filter. This will print the hint for the correct CDN in case someone uses a custom CDN.

Props peterwilsoncc.
Fixes #37387.

git-svn-id: https://develop.svn.wordpress.org/trunk@38122 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/general-template.php

@@ -2802,11 +2802,18 @@ function wp_site_icon() {
 function wp_resource_hints() {
 	$hints = array(
 		'dns-prefetch' => wp_dependencies_unique_hosts(),
-		'preconnect'   => array( 's.w.org' ),
+		'preconnect'   => array(),
 		'prefetch'     => array(),
 		'prerender'    => array(),
 	);
 
+	/*
+	 * Add DNS prefetch for the Emoji CDN.
+	 * The path is removed in the foreach loop below.
+	 */
+	/** This filter is documented in wp-includes/formatting.php */
+	$hints['dns-prefetch'][] = apply_filters( 'emoji_svg_url', 'https://s.w.org/images/core/emoji/2/svg/' );
+
 	foreach ( $hints as $relation_type => $urls ) {
 		/**
 		 * Filters domains and URLs for resource hints.