Implement isAudioOnly for src=

This is a generic implementation that doesn't rely on the audioTracks
and videoTracks members currently only available on Safari.

Issue #997

Change-Id: I849845513efb2ee51205dcdca8568c889f1f7cdb

lib/player.js

@@ -2561,8 +2561,14 @@ shaka.Player.prototype.isAudioOnly = function() {
     // Therefore if the first variant has no video, that's sufficient to say it
     // is audio-only content.
     return !variants[0].video;
+  } else if (this.video_ && this.video_.src) {
+    // 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
+    // true.
+    const video = /** @type {HTMLVideoElement} */(this.video_);
+    return video.videoHeight == 0;
   } else {
-    // TODO: Safari's native HLS has audioTracks/videoTracks on the element.
     return false;
   }
 };