If you are playing media (displaying a slideshow full screen, or playing a video) your screen will not sleep anyway. You might find this useful though if you sit on a webpage that is not playing media for quite a while and don't interact with it.
Once installed, pin the extension to your toolbar so it always appears at the top of your chrome browser.
Click the icon to enable keep-awake mode:
Moon = default (sleep when inactive)
Sun = always keep screen on
Sunset = you don't need this mode
This introduces some risk of accidentally leaving your computer unlocked since it will not sleep automatically. When in sun mode, ensure you use search+l key combo (or the power menu) to manually lock your screen anytime you step away to prevent unauthorized use of your account.
Video demo here: