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 3a6e169e4fc85b7a4426513d56dd51040aca1c7b..6eaee48d9333d144faf8967294d2e32f854f2dc8 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;
@@ -54,16 +53,19 @@ public:
 
     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