I don't believe that is part of the G+ API. If you really want to detect the outline of the face, look into some other API's that do such.
Here's a list of several face detection API's http://blog.mashape.com/post/53379410412/list-of-40-face-detection-recognition-apis
If you're dead set on using G+ the photo's would probably have to already have the face tagged. Then you'd have to rely on some sort of scrapping method to get that javascript code and decode/parse it.
You'd be better off using a face detection API.