sencha touch 2中判断游览器是否包含webkit的方法

<script type="text/javascript">
        if (!Ext.browser.is.WebKit) {
            alert("The current browser is unsupported.\n\nSupported browsers:\n" +
                "Google Chrome\n" +
                "Apple Safari\n" +
                "Mobile Safari (iOS)\n" +
                "Android Browser\n" +
                "BlackBerry Browser"
            );
        }
    </script>