private void BackButton_Click(object sender, RoutedEventArgs e)
{
if (News.IsSelected && Webview.CanGoBack && Webview.Source != new Uri("http://news.tsinghua.edu.cn/publish/thunews/index.html"))
{
Webview.GoBack();
}
}
public void OnClickLeft()
{
webview.GoBack();
}