chromium: Use system libvpx for version 25.

Unfortunately, we have build errors for version 25 in the bundled libvpx:

http://hydra.nixos.org/build/4173075
http://hydra.nixos.org/build/4173066

As I can't reproduce this on my local system (I've disabled the option
CONFIG_CC_STACKPROTECTOR here), let's just hope that libvpx is the only part
that fails during build because of this.

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