Only button with label has linked style.
var button_box = new Gtk.Box (Gtk.Orientation.HORIZONTAL, 0);
button_box.get_style_context ().add_class (Gtk.STYLE_CLASS_LINKED);
info_button = new Gtk.Button.from_icon_name ("dialog-information-symbolic", Gtk.IconSize.LARGE_TOOLBAR);
button_box.add (process_info_buttoninfo_button);
button_with_label = new Gtk.Button.with_label (_("Test"));
button_box.add (button_with_label);
header_bar.pack_start (button_box);