message: [Grappler] Expose empty constructor for MutationNewNode in MutableGraphView.
diff: @@ -244,15 +244,18 @@ class MutableNodeView
  };
  class MutationNewNode {
  + public:
  + MutationNewNode() {}
  +
  private:
  explicit MutationNewNode(Mutation* mutation, int mutation_counter, int index)
  : mutation_(mutation),
  mutation_counter_(mutation_counter),
  index_(index) {}
  - const Mutation* mutation_;
  - const int mutation_counter_;
  - const int index_;
  + Mutation* mutation_ = nullptr;
  + int mutation_counter_ = internal::kMissingSlot;
  + int index_ = internal::kMissingIndex;
  friend class Mutation;
  };
  