This prompt is actually appearing on the Google home page. Sounds like you have your homepage set to open Google as the first thing you see.
If you don't want the Chrome advertisement, you'll need to make a different page your homepage. To do that, open Safari, and then click on the Safari menu at the top left of the screen (next to the apple). Choose Preferences, and in the space that says Homepage, type the address of some other page. Or, just go to the website you want to be your homepage, and then open the Safari menu...instead o typing in the address, you can just click on Set To Current Page.