If there is glass between the outside and your cage (ie the window is closed) then yes keep your uvb light on. UVB cannot pass through glass or plastic, so while it will be good for keeping your temps right, it will do nothing for uvb.
Now, if the window is open, then you can turn your uvb light off for the few hours that the natural sunlight hits your cham, so long as your cham is in a mesh cage, not a glass/plastic one.
Natural sunlight is always better for chams than the uvb bulbs.
Remember, though, if your window is open, it will affect the temps in your cage.