chromium: Link against pulse instead of dlopen().

This fixes the issue of Chromium not being able to load the pulseaudio
librarp

We could also propagate the build inputs, but it would end up being the
same as just directoly linking against the library.

Thanks to @aristidb for noticing this in #2421:

https://github.com/NixOS/nixpkgs/pull/2421#issuecomment-42113656

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
(cherry picked from commit 17807c8b6cb7fd9f50bce98bb4557e202c571325)
chromium/source: Propagate system attribute.

The system attribute was already there in the function head of the
shared update helper but it actually wasn't used and thus later the
import of <nixpkgs> was done using builtins.currentSystem instead of the
system attribute inherited from the source derivation.

Now we correctly propagate the attribute, so that even when running a
64bit kernel you can run a 32bit Chromium with binary plugins.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
(cherry picked from commit 4f3085d5f8916eb1afebdddda4e9bfe675b29c52)