We use Gentoo's portage (aka emerge) as the package manager in Chromium OS. This page is more geared towards developers of the portage tool itself rather than developers just using it (i.e. for ebuilds, or configuration, etc...).
You can find Chromium OS's mini-fork here:
Changes constantly flow back into upstream Gentoo's tree, but we backport fixes sometimes.
The upstream repo can be found here: http://git.overlays.gentoo.org/gitweb/?p=proj/portage.git
You can add it to your local tree:
git remote add --tags upstream git://git.overlays.gentoo.org/proj/portage.git
Then you can view/cherry pick easily from there.
We use the master branch for our main development. When doing a major upgrade, the next branch can be used for that.