chromium: Minimal build (no install) from source.

This only gets chromium to build so far, installation is missing by upstream, so
we need to manually copy the corresponding files. And I guess with nix, we also
need to patch a few paths on installation.

Another issue is that at the moment, a lot of dependencies are used from the
source tree, rather than from the system.

Also, it would be nice to build using LLVM, as it really speeds up compilation a
*LOT* and also has the side effect of resulting in smaller binaries.

Working unit tests would be nice, too. Unfortunately they're quite heavyweight
and take hours to run, so I guess "someday" would be the most appropriate time
to integrate.

Further todo's:

- Allow to disable GConf, GIO and CUPS.
- Option to disable the sandbox (for whatever reason the user might have).
- Integrate gold binutils.
- Pulseaudio support.
- Clearly separate Linux specific stuff.