message: Further review comments.
diff: @@ -168,8 +168,8 @@ class Block {
  FloatingPoint block_size() const { return block_size_; }
  bool has_data() const { return has_data_; }
  - std::bitset<Update::kCount> updated() const { return updated_; }
  + const std::bitset<Update::kCount>& updated() const { return updated_; }
  std::bitset<Update::kCount>& updated() { return updated_; }
  bool& has_data() { return has_data_; }
  