@Override
  public long heapSize() {
    long ret = ClassSize.ARRAYLIST;
    for (Cell cell : cells) {
      ret += cell.heapSize();
    }
    return ret;
  }
Reads WALEdit from cells.
@param cellDecoder   Cell decoder.
@param expectedCount Expected cell count.
@return Number of KVs read.
hbase-server/src/main/java/org/apache/hadoop/hbase/wal/WALEdit.java