It turns out bug #319331 also appeared on a system with more recent nvidia-drivers. It seems I had to disable gpu rendering, and that's exactly what I have done. After you sync your tree, every www-client/chromium ebuild should contain the fix. Yes, remember to sync the tree. The ebuilds have been modified in-place. This way if you were not affected by this issue, you don't need to waste time to compile chromium again.
Unfortunately, I couldn't test with nvidia-drivers, but by grepping the build log I've verified that the file which caused the build failure is no longer compiled, so things should be fine now.
I'd like to add that I've noticed some reports about the issue on Gentoo Forums. However, the forums case was very ambiguous. The conclusion seemed to be that old nvidia-drivers make it break, but more recent drivers work fine. The problem with forums is that the communication there is unstructured and informal. A bug needs to have much more details, and a clearly set status. It's another bug reported that made me work around the issue on the Gentoo side.
So... if you hit any problems, please report bugs. An unreported bug is an unfixed bug.