Their support for rendering JavaScript is, IMO, that they have turned Chrome's engine into their web crawler.

We see some signs of this with the chrome headless project and with the fact that Chrome, masquerading as a mobile viewport, can be used as an effective mobile crawler, even when the mobile UX is provided primarily by JavaScript.

I think they still use a plain HTTP request based crawler for "most" sites (mainly for speed), but then flip on Chrome-based crawling for popular sites and for sites that seem to be JS-heavy. I see no reason why, long term, Chrome wouldn't become the primary crawl/render engine for Google.