Found by Viva64. Seems to have been a copy-paste error.

FWIW, this code isn't tested or run without the layout.css.clip-path-shapes.enabled pref set, and in any case since we don't have an actual implementation of the feature yet (just the CSS parsing) just having the values listed in property_database.js isn't sufficient to check that we store the values in the nsStyleClipPath correctly. I hear someone is (planning on?) working on CSS Masking (which I assume would encompass implementing the new clip-path values too) so hopefully we'll get test coverage from that soon.