Let me start with a general tip: if you're using Gentoo and hit a bug, please make sure there is a bug filed on Gentoo Bugzilla. Feel free to also report bugs upstream (this is great; remember to paste the link to the upstream bug in the Gentoo bug), but having a bug on the Gentoo side allows us to work around or at least be aware of issues for example when stabilizing new versions.
For example, see bug #371931: the upstream issue 78644 has been reported in April, but I only became aware of it late June, about 2 months of delay! Note that upstream generally prioritizes fixes that affect Google Chrome users, and not distro users. The bug does not affect Google Chrome, because it uses bundled ffmpeg. Upstream still wants to fix it (at some point they'll have to update ffmpeg), but it's just not a priority.
Okay, so let's review remaining bugs. If you want to help, this is a good opportunity.
bug #348841 - www-client/chromium: webgl not working: does http://bodybrowser.googlelabs.com/ work for you? Please post both success and failure reports, including emerge --info, www-client/chromium version and USE flags, media-libs/mesa version and USE flags. We may be able to correlate the reports and see what triggers problems.
bug #350250 - www-client/chromium-9.0.597.19 uses bundled protobuf: we still use bundled protobuf, and I'm afraid the version bundled with chromium may be patched in an incompatible way; I'll take a more detailed look when I find some time, but if you're interested in tinkering with the browser internals, it may be an interesting project; feel free to share your findings on that bug
bug #355181, bug #365841: we have some build issues with libpng-1.5; if you'd like to help fix them, please test with www-client/chromium-9999 and see if there are some not yet reported compile errors; if in doubt, just file a new bug; if you find upstream bugs or patches about this, please also report them
bug #361461 - www-client/chromium-11.0.696.25: fails to build with gcc 4.6; similar to the above: if you want to test, please use chromium-9999 and report any not yet reported issues, ideally both on the Gentoo and upstream side
bug #365187 - =www-client/chromium-11.0.696.57 and other version does not respect CFLAGS; this is a minor issue but it would be nice to fix it; if you're interested in modifying the upstream's custom build system, patches are welcome
bug #366695 - www-plugins/gecko-mediaplayer-1.0.3-r1 causes www-client/chromium to hang; this is sad - it seems many people would like to use gecko-mediaplayer, but we keep hitting various issues with it and Chromium; I'm not sure what's going on, so comments are welcome
bug #372495 - www-client/chromium fails to build with glibc-2.14 (tcmalloc); I think I'll just backport the upstream patch after some testing
That's it! Short bug queues are good for everyone, if you're interested feel free to read my earlier post.