I'm the author of the feature & blog post, it wasn't mean to hit HN. This is meant for gecko developers right now as it only measures the compositing GPU time, and not WebGL GPU time (different thread+Context). If this proves to be useful for compositing time then we will likely discuss this with the DevTools about integrating it with the new performance tools being developed.
We're also working on exposing disjoint timer query directly to WebGL to let people profile exactly what they need [1].
[1] https://bugzilla.mozilla.org/show_bug.cgi?id=974832