Fix isAudioOnly race with native HLS

With native HLS, we have a better, more accurate, and more timely way
to detect audio-only content.  Use that whenever possible.  This fixes
the audio-only poster and PiP element state with native HLS content.

Issue #997

Change-Id: Iee9f03deae6e56e7cfc140ed12278270eb667a41

lib/player.js

@@ -2598,6 +2598,14 @@ shaka.Player.prototype.isAudioOnly = function() {
     // is audio-only content.
     return !variants[0].video;
   } else if (this.video_ && this.video_.src) {
+    // If we have video track info, use that.  It will be the least error-prone
+    // way with native HLS.  In contrast, videoHeight might be unset until
+    // the first frame is loaded.  Since isAudioOnly is queried by the UI on
+    // the 'trackschanged' event, the videoTracks info should be up-to-date.
+    if (this.video_.videoTracks) {
+      return this.video_.videoTracks.length == 0;
+    }
+
     // We cast to the more specific HTMLVideoElement to access videoHeight.
     // This might be an audio element, though, in which case videoHeight will
     // be undefined at runtime.  For audio elements, this will always return