LAS VEGAS, NEV. — If you use Gmail and haven’t yet taken advantage of a feature Google unveiled last week to prevent hackers from hijacking your inbox, now would be an excellent time to do that. A security researcher at the Defcon hacker conference in Las Vegas on Saturday demonstrated a tool he built that allows attackers to break into your inbox even if you are accessing your Gmail over a persistent, encrypted session (using https:// versus http://). When you log in to Gmail, Google’s servers will place what’s called a “session cookie,” or small text file, on your machine. The cookie identifies your machine as having presented the correct user name and password for that account, and it can allow you to stay logged in to your account for up to two weeks if you don’t manually log out (after which the cookie expires and you are forced to