Change "invalid value default" for HTMLTrackElement 'kind' to "metadata"

third_party/WebKit/Source/core/html/HTMLTrackElement.cpp

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 76 matching lines @@
 {
     if (name == srcAttr) {
         if (!value.isEmpty())
             scheduleLoad();
         else if (m_track)
             m_track->removeAllCues();
 
     // 4.8.10.12.3 Sourcing out-of-band text tracks
     // As the kind, label, and srclang attributes are set, changed, or removed, the text track must update accordingly...
     } else if (name == kindAttr) {
-        track()->setKind(value.lower());
+        track()->setKind(!value.isNull() ? value.lower() : TextTrack::subtitlesKeyword());
     } else if (name == labelAttr) {
         track()->setLabel(value);
     } else if (name == srclangAttr) {
         track()->setLanguage(value);
     } else if (name == idAttr) {
         track()->setId(value);
     }
 
     HTMLElement::parseAttribute(name, oldValue, value);
 }
@@ skipping 217 matching lines @@
 }
 
 DEFINE_TRACE(HTMLTrackElement)
 {
     visitor->trace(m_track);
     visitor->trace(m_loader);
     HTMLElement::trace(visitor);
 }
 
 } // namespace blink