Gmail now has a recurring pop-up ad that asks you switch to Google Chrome—is there a setting/Safari extension I can use so that I never see it again?

[BlueberryMac]

When I go to Gmail in Safari, a little pop-up ad appears in the upper right-hand corner saying something like "Switch your default browser to Chrome" and only has a Yes or No option—no "Never ask again" option.

It's annoying to try and check email, but first have to click through this ad each time.

Ideas on how to kill the ad?