Tweak the TextTrackCue "cue index" management

Source/core/html/track/TextTrackCueList.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 26 matching lines @@
 class TextTrackCueList final : public RefCountedWillBeGarbageCollected<TextTrackCueList>, public ScriptWrappable {
     DECLARE_EMPTY_DESTRUCTOR_WILL_BE_REMOVED(TextTrackCueList);
     DEFINE_WRAPPERTYPEINFO();
 public:
     static PassRefPtrWillBeRawPtr<TextTrackCueList> create()
     {
         return adoptRefWillBeNoop(new TextTrackCueList);
     }
 
     unsigned long length() const;
-    unsigned long getCueIndex(TextTrackCue*) const;
 
     TextTrackCue* item(unsigned index) const;
     TextTrackCue* getCueById(const AtomicString&) const;
 
     bool add(PassRefPtrWillBeRawPtr<TextTrackCue>);
     bool remove(TextTrackCue*);
 
     void collectActiveCues(TextTrackCueList&) const;
     void updateCueIndex(TextTrackCue*);
+    bool isCueIndexValid(unsigned probeIndex) const { return probeIndex < m_firstInvalidIndex; }
+    void validateCueIndexes();
 
     DECLARE_TRACE();
 
 private:
     TextTrackCueList();
     size_t findInsertionIndex(const TextTrackCue*) const;
+    void invalidateCueIndex(size_t index);
     void clear();
-    void invalidateCueIndexes(size_t);
 
     WillBeHeapVector<RefPtrWillBeMember<TextTrackCue>> m_list;
+    size_t m_firstInvalidIndex;
 };
 
 } // namespace blink
 
 #endif // TextTrackCueList_h