chromium: Add support for ppapi flash and PDF.

This is hardcoded for the dev channel at the moment and we're going to
fetch it along with the main Chromium sources.

Also I'm putting this in default.nix at the moment, because we're going
to tear apart the whole Chromium package into several subparts soon.

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