Prompt the user before leaving the Customizer if they have unsaved changes. props westonruter. fixes #25439.

git-svn-id: https://develop.svn.wordpress.org/trunk@29025 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/theme.php

@@ -1874,6 +1874,9 @@ function _wp_customize_loader_settings() {
 		'url'           => esc_url( admin_url( 'customize.php' ) ),
 		'isCrossDomain' => $cross_domain,
 		'browser'       => $browser,
+		'l10n'          => array(
+			'saveAlert' => __( 'The changes you made will be lost if you navigate away from this page.' ),
+		),
 	);
 
 	$script = 'var _wpCustomizeLoaderSettings = ' . json_encode( $settings ) . ';';