Here is the code I've used, based on the original question and on Dominic P's answer...

    /*
     * Modify HTTP header
     */
    function add_header_xua() {
        if(!is_admin()) {
            header('X-UA-Compatible: IE=edge,chrome=1');    
        }
    }
    add_filter('wp_headers', 'add_header_xua');

Once you've added that code to your functions.php file, you can check it works by running a test at http://web-sniffer.net/ to ensure the HTTP headers have indeed changed.