I'm always more concerned about ultra-fine, expensive chocolate MELTING rather than freezing. That said, cargo compartments on passenger jets are pressurized (and thus, necessarily, cooled) the same as the cabins. But the other day, it was in the mid-90s at LAX (set a record); that kind of heat on the tarmac can melt fine chocolate in a hurry.

I'd be inclined to carry it on rather than risk theft by screener/baggage handler.