I'm pretty sure glass in general is fairly UV resistant, it's why the peroxide (toy whitening) treatment is slower when under glass, and why you only get driver arm with the window down.
It's not %100 but I'd say it's relatively good. obviously a treatment would improve it but may also make the glass harder to see through.
here is an interesting page
In my personal experience, direct sunlight is probalby the biggest problem with yellowing of toys. second is age.