Spray 4- to 5-quart slow cooker with cooking spray. In cooker, place garlic and oil. Sprinkle chicken with salt and pepper; place in cooker over garlic. Place mushrooms over chicken; pour wine over all.
Cover; cook on Low heat setting 5 to 6 hours.
Remove chicken from cooker; place on plate and cover to keep warm. In small bowl, mix water and cornstarch until smooth; stir into liquid in cooker. Increase heat setting to High; cover and cook about 10 minutes or until sauce is slightly thickened.
Return chicken to cooker. Cover; cook on High heat setting 5 minutes longer or until chicken is hot.
To serve, spoon mushroom mixture over chicken breasts; sprinkle with parsley.