chromium: Don't use v8 from <nixpkgs> anymore.

The version of v8 to use for Chromium is heavily tied to the specific
version of Chromium and thus it doesn't really make sense to use v8 from
<nixpkgs>, as we would need to have 3 different versions of v8, one for
each Chromium channel.

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