gnome-shell extension to add a "Reboot" button to the end-session-dialog, so to run grub-reboot before restarting the system, in order to reboot your computer to the selected operating system.
This extension needs permissions for gnome-shell to read your grub.cfg file, please verify your Linux distribution documentation.
This extension is a GUI for the utility grub-reboot. You may want to set your grub configuration properly before using the extension, this stackexchange might prove useful. The standard Ubuntu configuration should work just fine though.
When you select the operating system to reboot into, you'll be required to input your password.