Heat the oil in a large, heavy saucepan over medium heat. Cook and stir the chicken until browned, about 3 minutes. Add potatoes and onion; cook and stir until browned, 3 to 5 minutes more. Add stock, green chillies, zucchini, carrots, salt and garlic salt. Bring to a boil; reduce heat and simmer until vegetables are tender, about 1 hour. Add corn and simmer until heated through, 10 to 15 minutes more.