MSE: Implement TrackDefault object

Source/core/html/track/VideoTrack.h

Index: Source/core/html/track/VideoTrack.h
diff --git a/Source/core/html/track/VideoTrack.h b/Source/core/html/track/VideoTrack.h
index 678a17ebbfca004443a7e838b2a9b4b54c611da6..34958c4ee1ac5a02ee8dd70628f284807d653323 100644
--- a/Source/core/html/track/VideoTrack.h
+++ b/Source/core/html/track/VideoTrack.h
@@ -34,11 +34,13 @@ public:
     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;