Tweak the TextTrackCue "cue index" management

Source/core/html/track/TextTrackCueList.h

Index: Source/core/html/track/TextTrackCueList.h
diff --git a/Source/core/html/track/TextTrackCueList.h b/Source/core/html/track/TextTrackCueList.h
index 0f11f2b60c2de7ec35dd749a0dda0211619a59a4..e962c85e1ae8ccc2068accb25edfb0d00ff7fc3f 100644
--- a/Source/core/html/track/TextTrackCueList.h
+++ b/Source/core/html/track/TextTrackCueList.h
@@ -44,7 +44,6 @@ public:
     }
 
     unsigned long length() const;
-    unsigned long getCueIndex(TextTrackCue*) const;
 
     TextTrackCue* item(unsigned index) const;
     TextTrackCue* getCueById(const AtomicString&) const;
@@ -54,17 +53,20 @@ public:
     bool remove(TextTrackCue*);
 
     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;
     RefPtrWillBeMember<TextTrackCueList> m_activeCues;
+    size_t m_firstInvalidIndex;
 };
 
 } // namespace blink