And by "Stable" the headline actually means "Canary" at this point. It will be a while before it reaches the stable version, but it's coming. This is good news.
I am running normal Chrome on Mac (Version 49.0.2623.87) and the feature flag is there under chrome://flags/
As LukeAl said in the post you are commenting on, the stable version's native notification feature is 6-12 weeks behind Canary's (which the chromium.org update is talking about).