If you wonder how it's possible that Chromium upstream released 6.0.422.0 dev channel release, but the version in portage is stuck at 6.0.408.1, the answer is simple: at some point the upstream-generated tarballs became broken. They don't contain important bits of WebKit, and the browser can't be compiled. In fact, even the Makefiles can't be generated.
And how is it possible that the tarballs are broken for so long? The normal release process only produces binary packages. Tarballs are produced automatically, and the only people who care about them are Linux distro packagers (that includes me). I'm working to get this fixed in a proper way upstream.
So you'll have to wait a bit for the next dev channel release in Gentoo... However, when we finally have working tarballs, I'm going to make some little improvements to the ebuilds. Just to keep our Chromium package even better.