When people say a horse has "chrome," they obviously don't literally mean it has, say, metal hooves.
Chrome is white markings - most especially stockings or socks - that are considered particularly attractive or flashy. A horse with "a lot of chrome" probably has high white stockings and often a blaze as well. You might also hear people say that certain lines "don't tend to give much chrome."
The term is US specific and relatively recent in coinage.
With his three white legs, belly spots and broad stripe/narrow blaze, you might accuse this Clydesdale of having "plenty of chrome." (Source: USDA by Bill Tarpenning).