As far as I know, google has changed its re-captcha to a new one for google chrome browser. Google URL Shortener uses this kind of captcha.
This re-captcha verifies that "We are not robot" automatically only with a single click. But how does it work?
In the image below, you can see the captcha.
(1) We click on "I'm not a robot" and (2) after a while, (3) re-captcha verifies that automatically: