title: getUserMedia() in PWA on iOS 11.3.1
body: <p>I'm trying to build a PWA of my webapp. My webapp makes use of getUserMedia to let the user make a picture. the webapp works as expected. the browser asks permission to access the camera and if a user accepts it, the app continues to work. Now, I made a PWA which works except for the camera. The user doesn't get a prompt to give access to its camera which is where I think is the problem. Is there any way to trigger the camera access: </p>
  
  <pre><code>   let constraints = {
         video: {
             facingMode: "user"
         },
         audio: false
     };
  
  navigator.mediaDevices
      .getUserMedia(constraints)
      .then(function (stream) {
          track = stream.getTracks()[0];
          cameraView.srcObject = stream;
      })
      .catch(function (error) {
          console.error("Oops. Something is broken.", error);
      });
  </code></pre>
  
  <p>like such inside a PWA? or is this not possible (yet)? If not, are there any plans for PWA's on iOS to have this functionality?</p>