Google can't seem to decide on a chat protocol and stick with it for any length of time, but SMS has been around for years and will continue to exist long into the future. The problem: it's tied to your phone. Some third-party apps can sync notifications to your computer, but Google looks to be preparing a built-in SMS connection for Chrome OS.
The new "Connected Devices" setting has been popping up on some devices seemingly at random. One AP staffer saw the new menu, but it was just an empty box. At least some users are seeing the full listing, which explains "SMS Connect" will allow you to get text message notifications on your Chromebook.
The mechanism for this is unclear. Perhaps it will be tied into Google's own Android Messages app? There doesn't appear to be any way to force this option to show up—even toggling various flags that might be related produces no result. Being on the dev channel won't help, either. Only time will tell.
- Chrome Unboxed