calling getVideoPlaybackQuality() during seek will cause hang instantly until end of downloads media source

calling getVideoPlaybackQuality() during seek will cause hang instantly until end of downloads media source.
This also happened when Web Inspector is opened.

* Reproducing step

1. play any mp4 video with url that is never cached yet
2. registering `seeking` event listener which contains getVideoPlaybackQuality()
3. move playhead to unbuffered timeline on video timeline shown
4. repeat 3 until hang it browser

(optionally) If you cannot reproduce with above steps, maybe using `timeupdate` handler will helpful to reproduce (see to code stubs in below)


* Reproducing environments

iOS Safari / WKWebView (iOS 12.1, iOS 12.2, iOS 12.3 public beta3)
macOS Safari 12.1.2, Safari Technical Preview 81 (macOS 10.14.4)
MiniBrowser.app (r244898, macOS 10.14.4)


* Test stubs

<!--
you need to change video source, and it should be MP4
Also, It is not matter of video container.
-->
<video src="http://example/v.mp4" controls id="x"></video>
<div id="logger"></div>

<script>
  var x = document.getElementById('x');
  var l = document.getElementById('logger');
  function log(
     l.innerHTML = object.totalVideoFrames;
  }

  x.addEventListener('seeking', function () {
    log(x.getVideoPlaybackQuality());
  });
  // It happened more frequently happened with following stubs
  // x.addEventListener('timeupdate', function () {
  //  log(x.getVideoPlaybackQuality());
  // });
 </script>

[Radar WebKit Bug Importer]

<rdar://problem/50447868>