Revert of Simplify LoadableTextTrack::trackElementIndex

third_party/WebKit/Source/core/html/track/LoadableTextTrack.h

 /*
  * Copyright (C) 2011 Google 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 30 matching lines @@
         return new LoadableTextTrack(track);
     }
     ~LoadableTextTrack() override;
 
     // TextTrack method.
     void setMode(const AtomicString&) override;
 
     void addRegions(const HeapVector<Member<VTTRegion>>&);
     using TextTrack::addListOfCues;
 
-    size_t trackElementIndex() const;
+    size_t trackElementIndex();
     HTMLTrackElement* trackElement() { return m_trackElement; }
 
     bool isDefault() const override;
 
     DECLARE_VIRTUAL_TRACE();
 
 private:
     explicit LoadableTextTrack(HTMLTrackElement*);
 
     Member<HTMLTrackElement> m_trackElement;
 };
 
 } // namespace blink
 
 #endif // LoadableTextTrack_h