As you have probably noticed, www-client/chromium ebuilds are masked in an interesting way. After the latest beta channel release the situation is finally clear. If you want to be on the dev channel, you have to hard unmask the package. If you want to be on the beta channel, just ~keyword unmask it. There are no stable releases yet. For more information please refer to http://dev.gentoo.org/~phajdan.jr/ and http://googlechromereleases.blogspot.com/.