Due to popular demand, I've added experimental support for ChromeDriver to the latest dev channel release of www-client/chromium. It is controlled by USE="chromedriver".
Please report any issues, even minor ones, with this flag. This should work "out of the box", without any additional downloads or setup. If it doesn't, I'd like to fix it.
By the way, that dev channel release also has optional support for PulseAudio. It is controlled by USE="pulseaudio".
And there are only 4 open bug reports assigned to the Gentoo Chromium Team!