Use TextTrackMode and TextTrackKind enums in TextTrack IDL

Source/core/html/track/TextTrack.cpp

 /*
  * Copyright (C) 2011 Google Inc.  All rights reserved.
  * Copyright (C) 2011, 2012, 2013 Apple Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 144 matching lines @@
         m_kind = kind;
     else
         m_kind = subtitlesKeyword();
 
     if (m_client && oldKind != m_kind)
         m_client->textTrackKindChanged(this);
 }
 
 void TextTrack::setMode(const AtomicString& mode)
 {
+    ASSERT(mode == disabledKeyword() || mode == hiddenKeyword() || mode == showingKeyword());
+
     // On setting, if the new value isn't equal to what the attribute would currently
     // return, the new value must be processed as follows ...
-    if (mode != disabledKeyword() && mode != hiddenKeyword() && mode != showingKeyword())
-        return;
-
     if (m_mode == mode)
         return;
 
     // If mode changes to disabled, remove this track's cues from the client
     // because they will no longer be accessible from the cues() function.
     if (mode == disabledKeyword() && m_client && m_cues)
         m_client->textTrackRemoveCues(this, m_cues.get());
 
     if (mode != showingKeyword() && m_cues)
         for (size_t i = 0; i < m_cues->length(); ++i)
@@ skipping 310 matching lines @@
     return EventTargetNames::TextTrack;
 }
 
 ExecutionContext* TextTrack::executionContext() const
 {
     return m_document;
 }
 
 } // namespace WebCore