Swift - 네 비게 이 션 표시 줄 '되 돌리 기' 단추 텍스트, 아이콘 수정

http://www.hangge.com/blog/cache/detail_957.html

        let leftBarBtn = UIBarButtonItem(title: "", style: .plain, target: self,action: #selector(backItemPressed))
        leftBarBtn.image = UIImage(named: "    ")
        //        ，           
        let spacer = UIBarButtonItem(barButtonSystemItem: .fixedSpace, target: nil,action: nil)
        spacer.width = -10;
        self.navigationItem.leftBarButtonItems = [spacer, leftBarBtn]

    func backItemPressed() {
        if webView.canGoBack {
            webView.goBack()
        }else{
            if let nav = self.navigationController {
                nav.popViewController(animated: true)
            }
        }
    }