How can I use Optimus to render Youtube only when plugged in?

Question

I have a laptop with an Nvidia Optimus card. I'd like to use the discrete graphics to render Youtube videos when I'm plugged in, but I want to switch back to integrated graphics on battery power. So far, the only way I can think of to make Youtube render on discrete graphics is to set Optimus to render Chrome with discrete graphics, but then I think it would do so even on battery.