Themes: Exit `wp-custom-header.js` early in browsers wihout modern events.

Adds a check for `window.addEventListener` support before attempting to use it. In browsers without support, the file is exited early to avoid throwing a JavaScript error.

Fixes #38585.


git-svn-id: https://develop.svn.wordpress.org/trunk@39050 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/js/wp-custom-header.js

@@ -1,5 +1,10 @@
 (function( window, $, settings ) {
 
+	if ( ! ( 'addEventListener' in window ) ) {
+		// Fail gracefully in unsupported browsers.
+		return;
+	}
+
 	function wpCustomHeader() {
 		var handlers = {
 			nativeVideo: {