title: swift 3 error: Type 'AVLayerVideoGravity' has no member 'resizeAspectFill'
body: <p>I am working on an iOS app using Swift 3 where I have a video playing in the background. My code was working completely fine until randomly the error message:</p>
  
  <blockquote>
    <p>Type <code>AVLayerVideoGravity</code> aka (<code>NSString</code>) has no member <code>resizeAspectFill</code></p>
  </blockquote>
  
  <p>appears. I can't figure out at all why resizeAspectFill is no longer being recognized. Is there something I'm missing? I've tried cleaning my project but that does not fix anything. Any help would be greatly appreciated :) 
  Below is the code: </p>
  
  <pre><code>import Foundation
  import AVFoundation
  import UIKit
  
  class HomeViewController: UIViewController {
  // MARK: Properties
  var avPlayer: AVPlayer!
  var avPlayerLayer: AVPlayerLayer!
  var paused: Bool = false
  
  var enterButton: UIButton! = UIButton()
  
  override func viewDidLoad() {
      super.viewDidLoad()
      self.title = "Welcome"
  
      let theURL = Bundle.main.url(forResource:"hannah", withExtension: "mp4")
  
      avPlayer = AVPlayer(url: theURL!)
      avPlayerLayer = AVPlayerLayer(player: avPlayer)
      avPlayerLayer.videoGravity = AVLayerVideoGravity.resizeAspectFill
  
      avPlayer.volume = 0
      avPlayer.actionAtItemEnd = .none
  
      avPlayerLayer.frame = view.layer.bounds
      view.backgroundColor = .clear
      view.layer.insertSublayer(avPlayerLayer, at: 0)
  
      NotificationCenter.default.addObserver(self,
                                             selector: #selector(playerItemDidReachEnd(notification:)),
                                             name: NSNotification.Name.AVPlayerItemDidPlayToEndTime,
                                             object: avPlayer.currentItem)
  </code></pre>