Place chicken in small bowl. Add soy sauce, garlic and black pepper; stir to coat. Marinate 15 minutes at room temperature.
Meanwhile, stir chicken broth and onion powder into cornstarch in small bowl until smooth; set aside.
Heat oil in wok over high heat. Reduce heat to medium-high. Add chicken; stir-fry 2 minutes. Add bell pepper and celery; stir-fry 2 minutes. Add corn; cook and stir 3 minutes.
Stir in cornstarch mixture; cook and stir over high heat until mixture boils and slightly thickens.