chromium: Add preliminary support for GN

This only uses the most basic GN flags which should represent the GYP
flags we had before. In order to get rid most of the GYP cruft, we now
have common.nix and common-gn.nix which are mostly the same, just that
the latter is only for GN builds.

The GN implementation is far from complete and currently not even
builds, so we need more work to get the beta and dev channels building.

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