title: subview not showing in scrollview when using visual format constraint
body: <p>I cannot understand why the blue view is not showing please take a look at my code. When i use <code>view.addSubView(mapView)</code> its shows in the main view but when I do <code>scrollView.addSubView(mapView)</code> it does not show.Everything appear to be correct with the constraint. I not sure why its not working.</p>
  
  <pre><code>import UIKit
  
  
  
  class practiceViewController: UIViewController,UIScrollViewDelegate {
      override func viewDidLoad() {
          var allConstraint = [NSLayoutConstraint]()
  
          let scrollView:UIScrollView = {
              let v = UIScrollView()
              v.backgroundColor = .red
              v.translatesAutoresizingMaskIntoConstraints = false
              return v
          }()
  
          let mapView:UIView = {
              let v = UIView()
              v.backgroundColor = .blue
              v.translatesAutoresizingMaskIntoConstraints = false
              return v
          }()
  
          //views
          let views = [
              "scrollView":scrollView,
              "mapView":mapView
          ]
          //add views
          view.addSubview(scrollView)
          scrollView.addSubview(mapView)
  
         // constraints
          let scrollviewHorizontalConstraint = NSLayoutConstraint.constraints(withVisualFormat: "H:|[scrollView]|", options: [], metrics: nil, views: views)
          let scrollviewVerticleConstraint = NSLayoutConstraint.constraints(withVisualFormat: "V:|[scrollView]|", options: [], metrics: nil, views: views)
  
          allConstraint += scrollviewVerticleConstraint
          allConstraint += scrollviewHorizontalConstraint
  
          let mapViewHorizontalConstraint = NSLayoutConstraint.constraints(withVisualFormat: "H:|[mapView]|", options: [], metrics: nil, views: views)
          let mapViewVerticleConstraint = NSLayoutConstraint.constraints(withVisualFormat: "V:|[mapView]|", options: [], metrics: nil, views: views)
  
          allConstraint += mapViewHorizontalConstraint
          allConstraint += mapViewVerticleConstraint
  
          NSLayoutConstraint.activate(allConstraint)
      }
  
  }
  </code></pre>