chromium: Add WideVine content decryption plugin.

Seems to be needed in order to view Netflix content, but not sure
whether it is sufficient by its own or whether you need an additional
Netflix plugin as well.

Of course, this plugin is disabled by default as well as all the other
proprietary plugins.

For the plugin derivation, we now do the checkPhase _after_ the
installPhase, to make sure we also detect RPATHs pointing to the plugin
directory itself, because the shared object files only exist after the
installPhase.

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