MSE: Implement TrackDefault object

Source/core/html/track/VideoTrack.cpp

 // 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.
 
 #include "config.h"
 #include "core/html/track/VideoTrack.h"
 
 #include "core/html/HTMLMediaElement.h"
 
 namespace blink {
@@ skipping 51 matching lines @@
     DEFINE_STATIC_LOCAL(const AtomicString, keyword, ("subtitles", AtomicString::ConstructFromLiteral));
     return keyword;
 }
 
 const AtomicString& VideoTrack::commentaryKeyword()
 {
     DEFINE_STATIC_LOCAL(const AtomicString, keyword, ("commentary", AtomicString::ConstructFromLiteral));
     return keyword;
 }
 
-bool VideoTrack::isValidKind(const AtomicString& kind) const
+bool VideoTrack::isValidKindKeyword(const String& kind)
 {
     return (kind == alternativeKeyword())
         || (kind == captionsKeyword())
         || (kind == mainKeyword())
         || (kind == signKeyword())
         || (kind == subtitlesKeyword())
-        || (kind == commentaryKeyword());
+        || (kind == commentaryKeyword())
+        || (kind == emptyAtom);
 }
 
 AtomicString VideoTrack::defaultKind() const
 {
     return emptyAtom;
 }
 
 }