message: Make ShapeTreeIterator's operator* and operator-> const
  They can be const because they don't mutate the iterator (even though they may give access to mutable data).
diff: @@ -385,8 +385,8 @@ class ShapeTreeIterator
  bool operator!=(const ShapeTreeIterator& other) const {
  return node_ != other.node_;
  }
  - ValueType& operator*() { return node_->data; }
  - ValueType* operator->() { return &node_->data; }
  + ValueType& operator*() const { return node_->data; }
  + ValueType* operator->() const { return &node_->data; }
  private:
  ContainerType* nodes_;
  