feat(ui): Allow showing track labels in UI

Instead of language, role, or both, it is sometimes useful for an
application to show the "label" field of the corresponding "Track"
object.

Closes shaka-project#2927

Change-Id: Ifc2fa8137bc73ec704443c6071f102805f70dc3d

ui/externs/ui.js

@@ -117,7 +117,10 @@ shaka.extern.UIVolumeBarColors;
  *   audio variant selection.
  *   LANGUAGE means that only the language of the item is shown.
  *   ROLE means that only the role of the item is shown.
- *   LANGUAGE_ROLE means both are shown, or just language if there is no role.
+ *   LANGUAGE_ROLE means both language and role are shown, or just language if
+ *   there is no role.
+ *   LABEL means the non-standard DASH "label" attribute or the HLS "NAME"
+ *   attribute are shown.
  *   Defaults to LANGUAGE.
  * @property {number} fadeDelay
  *   The delay (in seconds) before fading out the controls once the user stops

ui/language_utils.js

@@ -111,6 +111,16 @@ shaka.ui.LanguageUtils = class {
             span.textContent += ': ' + rolesString;
           }
           break;
+        case shaka.ui.TrackLabelFormat.LABEL:
+          if (track.label) {
+            span.textContent = track.label;
+          } else {
+            // Fallback behavior. This probably shouldn't happen.
+            shaka.log.alwaysWarn('Track #' + track.id + ' does not have a ' +
+                'label, but the UI is configured to only show labels.');
+            span.textContent = '?';
+          }
+          break;
       }
 
       if (updateChosen && (combinationName == selectedCombination)) {

ui/ui.js

@@ -417,6 +417,7 @@ shaka.ui.TrackLabelFormat = {
   'LANGUAGE': 0,
   'ROLE': 1,
   'LANGUAGE_ROLE': 2,
+  'LABEL': 3,
 };
 
 /**