If you'd rather not compile www-client/chromium (it takes some time, even on a powerful machine), give www-client/chromium-bin a try.
Now all versions in the tree are compiled on a Gentoo development machine, and synchronized with www-client/chromium releases. Special thanks go here to the Gentoo Infrastructure team for providing the machine for Chromium in Gentoo project.
The -bin version of the package is not stabilized yet, but I plan to start the process soon. Some testing before that happens would be appreciated. Please report all issues to the Gentoo Bugzilla.