MSE: Implement TrackDefault object

Source/core/html/track/VideoTrack.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef VideoTrack_h
 #define VideoTrack_h
 
 #include "bindings/core/v8/ScriptWrappable.h"
 #include "core/html/track/TrackBase.h"
 
@@ skipping 16 matching lines @@
     void clearSelected() { m_selected = false; }
 
     // Valid kind keywords.
     static const AtomicString& alternativeKeyword();
     static const AtomicString& captionsKeyword();
     static const AtomicString& mainKeyword();
     static const AtomicString& signKeyword();
     static const AtomicString& subtitlesKeyword();
     static const AtomicString& commentaryKeyword();
 
+    static bool isValidKindKeyword(const String&);
+
 private:
     VideoTrack(const String& id, const AtomicString& kind, const AtomicString& label, const AtomicString& language, bool selected);
 
     // TrackBase
-    virtual bool isValidKind(const AtomicString&) const override;
+    virtual bool isValidKind(const AtomicString& kind) const override { return isValidKindKeyword(kind); }
     virtual AtomicString defaultKind() const override;
 
     bool m_selected;
 };
 
 DEFINE_TRACK_TYPE_CASTS(VideoTrack, TrackBase::VideoTrack);
 
 } // namespace blink
 
 #endif // VideoTrack_h