Konqueror[1] can toggle between KHTML and Webkit at runtime.
GNOME Web (formerly Epiphany)[2] is Webkit based.
Surf[3] (By the suckless st/dwm people) is WebKit based.
That said, Surf would be unlikely to move to chromium from my perception of the views of the suckless group, and their opinions of how software would work are really opposed to mainstream appeal, Konqueror undoubtedly will eventually replace QtWebkit with QtWebEngine for at least the Webkit support (if not dropping KHTML entirely - KHTML's own readme suggests not using it), so that leaves Gnome Web as the one open source browser aimed at a wider audience using Webkit.
[1]: https://kde.org/applications/internet/org.kde.konqueror
Is there a maintained OSS browser based on Webkit? Do you think it's viable for a new Webkit-based OSS browser to become sustainable by user donations, assuming initial sponsorship by other non-profits or EU?