how to enable screen/desktop capture in chrome?
In Chrome/Chromium you cannot use "standard" Javascript to capture users screen anymore.
It was replaced by the chrome.desktopCapture API, which is exclusive for Chrome extensions.
The example you're providing rely in this API and an extensions. It's plugin free but not extension free.
The enable-usermedia-screen-capture flag was removed from chrome://flags/ in Chrome M36 (Chromium Issue 347641).
For testing or experimenting, the feature can still be enabled when starting Chrome with chrome --enable-usermedia-screen-capturing (tested in Chrome 68).
Screenshot:
The Chrome Platform Status also highlights a MediaStream.getDisplayMedia() implementation, which will allow developers to record or share the screen from the browser without any plugin. The specification for this can be found in the Screen Capture Editor's Draft.