Running content_shell for Fuchsia under the new packaged runner enables sandboxing. However, the namespace-building portion of child-process launching then fails while trying to create a handle for /pkg, to pass to the child process:

ERROR:launch_fuchsia.cc(90)] fdio_transfer_fd failed: ZX_ERR_NOT_SUPPORTED

This is reported for /pkg, suggesting that the directory's fd is of a type that we cannot unwrap into transferable handles, for some reason.