We cannot compile with just feature(gecko + debug_assertions), since that's how
debug rusttests get compiled and they don't have the refcount logging stuff.

We were getting away with it for the pre-existing usage of the style crate,
because it wasn't used during any test and presumably the linker didn't
complain. But servo_arc is definitely used in tests.

Differential Revision: https://phabricator.services.mozilla.com/D32691