MSE: Implement TrackDefault object

Source/core/html/track/AudioTrack.cpp

Index: Source/core/html/track/AudioTrack.cpp
diff --git a/Source/core/html/track/AudioTrack.cpp b/Source/core/html/track/AudioTrack.cpp
index ac8c92e96418b4e35fbd7741a6fe5b1277a771ba..ef994d09212c71c3ea1ef012e6f7d5bdb517af23 100644
--- a/Source/core/html/track/AudioTrack.cpp
+++ b/Source/core/html/track/AudioTrack.cpp
@@ -67,14 +67,26 @@ const AtomicString& AudioTrack::commentaryKeyword()
     return keyword;
 }
 
-bool AudioTrack::isValidKind(const AtomicString& kind) const
+bool AudioTrack::isValidKindKeyword(const AtomicString& kind)
 {
     return (kind == alternativeKeyword())
         || (kind == descriptionsKeyword())
         || (kind == mainKeyword())
         || (kind == mainDescriptionsKeyword())
         || (kind == translationKeyword())
-        || (kind == commentaryKeyword());
+        || (kind == commentaryKeyword())
+        || (kind == emptyAtom);
+}
+
+bool AudioTrack::isValidKindKeyword(const String& kind)
+{
+    return (kind == alternativeKeyword())
+        || (kind == descriptionsKeyword())
+        || (kind == mainKeyword())
+        || (kind == mainDescriptionsKeyword())
+        || (kind == translationKeyword())
+        || (kind == commentaryKeyword())
+        || (kind == emptyAtom);
 }
 
 AtomicString AudioTrack::defaultKind() const