chromium: Factor out common chromium attributes.

This results in a new function called mkChromiumDerivation, which can be
used to easily build packages that are based on the Chromium source
tree.

We pass through this function as mkDerivation in the chromium wrappre,
so in the end if you want to create such a package, something like:

chromium.mkDerivation (base: {
  name = "your-shiny-package-based-on-chromium";
  ...
})

will suffice.

Of course, this is only the first step towards this functionality,
because right now I'm not even sure the Chromium browser itself will
build.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>