MSE: Implement TrackDefault object

Source/core/html/track/VideoTrack.cpp

Index: Source/core/html/track/VideoTrack.cpp
diff --git a/Source/core/html/track/VideoTrack.cpp b/Source/core/html/track/VideoTrack.cpp
index 86d26b57cf077973b8ab39a406cdc856d6914768..18abe424f28fa2a6bbf30bf5813214f969d44536 100644
--- a/Source/core/html/track/VideoTrack.cpp
+++ b/Source/core/html/track/VideoTrack.cpp
@@ -69,14 +69,26 @@ const AtomicString& VideoTrack::commentaryKeyword()
     return keyword;
 }
 
-bool VideoTrack::isValidKind(const AtomicString& kind) const
+bool VideoTrack::isValidKindKeyword(const AtomicString& kind)
 {
     return (kind == alternativeKeyword())
         || (kind == captionsKeyword())
         || (kind == mainKeyword())
         || (kind == signKeyword())
         || (kind == subtitlesKeyword())
-        || (kind == commentaryKeyword());
+        || (kind == commentaryKeyword())
+        || (kind == emptyAtom);
+}
+
+bool VideoTrack::isValidKindKeyword(const String& kind)
+{
+    return (kind == alternativeKeyword())
+        || (kind == captionsKeyword())
+        || (kind == mainKeyword())
+        || (kind == signKeyword())
+        || (kind == subtitlesKeyword())
+        || (kind == commentaryKeyword())
+        || (kind == emptyAtom);
 }
 
 AtomicString VideoTrack::defaultKind() const