chromium: Properly implement mksnapshot target.

Let's ensure we do all architecture-dependant stuff inside
mkChromiumDerivation and not pass archInfo around, so we can properly
decouple it from the main function.

This partially reverts 8d54dc6d

.

The main reason for doing this is because the architecture information
is no longer required in Chromium 37, so let's uglify and XXX it in
common.nix and remove it once version 37 hits the stable channel.

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