@Override
  protected long indexEntryOnHeapSize(boolean onHeap) {
    if (onHeap) {
      return indexEntrySize();
    }
    // else the index is allocated off-heap
    return 0;
  }
------------------------------------------------------------------------ C-tor to be used when
new CellChunkImmutableSegment is built as a result of flattening of CSLMImmutableSegment The
given iterator returns the Cells that "survived" the compaction.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/CellChunkImmutableSegment.java