登录后怎么关闭登录webview

$.oldBack = mui.back;
var backButtonPress = 0;
$.back = function(event) {
backButtonPress++;
if (backButtonPress > 1) {
plus.runtime.quit();
} else {
plus.nativeUI.toast('再按一次退出应用');
}
setTimeout(function() {
backButtonPress = 0;
}, 1000);
return false;
};