If your invention is something that you can patent, then Google maintains a patent purchase program wherein you can put forward your patent in order for you to sell to them.
It's tough to convince someone at Google to listen to your proposal if it's just that—an idea. It would be hard to listen to everyone, but there are likely hundreds of ideas out there that people want to pitch to Google. Therefore, the only way to get your foot in the door is with a patent.
