I'm a security engineer at Facebook and this is my fault. We're testing this for some users to see if it can slow down some attacks where users are tricked into pasting (malicious) javascript into the browser console.
Just to be clear: trying to block hackers client-side is a bad idea in general; this is to protect against a specific social engineering attack.
If you ended up in the test group and are annoyed by this, sorry. I tried to make the opt-out page as simple as possible while still being scary enough to stop at least some of the victims.
The actual code is pretty similar to @joeldixon66's link; ours is a little more complicated for no good reason.
Chrome wraps all console code in
with ((console && console._commandLineAPI) || {}) {
<code goes here>
}
... so the site redefines console._commandLineAPI to throw:
Object.defineProperty(console, '_commandLineAPI',
{ get : function() { throw 'Nooo!' } })
This is not quite enough (try it!), but that's the main trick.