If you read here you can see that you can look for the OPR string to identify Opera.
http://my.opera.com/ODIN/blog/2013/07/15/opera-user-agent-strings-opera-15-and-beyond
For example you can include it in your test like this:
var userAgent = navigator.userAgent.toLowerCase();
var is_chrome = userAgent.indexOf('chrome') > -1 && userAgent.indexOf('opr/') == -1;