Just like bug 353466, the www-client/chromium emerge process invokes `svn info` which stalls when portage's build directory is using btrfs. Guilty: build/util/lastchange.py in chromium's tarball.
I agree that there is a problem (that we could workaround), but it's really not on the chromium side. The root cause seems to be btrfs, please try to get that fixed upstream.