This bug has been fixed: https://code.google.com/p/chromium/issues/detail?id=494939 Make libvpx native and make it depend on 1.5.
Relevat comment in the ebuild: # TODO: use_system_libvpx (http://crbug.com/494939).
Unfortunately there's another issue, https://code.google.com/p/chromium/issues/detail?id=575651 .
I have read the bug report and apparently the solution is: 1. Add the use flag "svc" to libvpx that enables: --enable-experimental --enable-spatial-svc 2. Make chromium depend on libvpx[svc]
libvpx now has the SVC flag, maybe try it out?
This should be fixed with https://gitweb.gentoo.org/repo/gentoo.git/commit/www-client/chromium?id=19050b84a294b9807bd6e755b75230141892f195 .