Gavin has similar white spots on his eye. A vet thought that it was corneal dystrophy, but didn't really know for sure. With Gavin we did a stain, and his eye was perfectly normal. The white spots have grown since then (this was 3 years ago), and we assumed that they might eventually impact his vision. However, he went blind from a completely unrelated illness (cataracts) so it's a moot point.
Osseous choristomas are classified as tumors, aren't they? Meg has those white spots in the middle of her eye. However, many of my old folks have depigmentation of the edge of the eye, which looks kind of like that photo. Meg has the most serious case -- so she always looks like she is surprised because there is white all around her pupils now. My vet says that this depigmentation is associated with age.