The Smart Lock feature that has been slowly cooking in the Chrome OS dev and beta channels has made its way into the latest stable release, version 40. Now anyone with a phone running Android 5.0 or later (sorry, no tablets) can automatically unlock their Chromebook just by keeping the two devices within 100 feet of each other.
You can find the option tucked away under advanced settings. In this shot I've scrolled the area to the top and have already turned things on.
Smart Lock is still in beta, but so far it works-ish. I find the feature to be amusing in practice. It only unlocks your computer if your phone is unlocked. That means instead of typing in my password, I lift my phone off the desk and swipe to get in. It's not as seamless as keeping my phone unlocked with a smartwatch, but it's still easier than typing out my password umpteen times a day.
Update: You can streamline the process by relying on either a trusted device (such as an Android Wear watch) or trusted place (such as your home) to keep your smartphone unlocked. In conjunction with either of these approaches, using Smart Lock on a Chromebook feels much more natural.
The process is pretty straightforward, but you can get more information on Google's support page.