message: Make pImpl class private.
diff: @@ -127,9 +127,9 @@ class ThreadPool {
  // pointer points to, and should not attempt to delete.
  Eigen::ThreadPoolInterface* AsEigenThreadPool();
  + private:
  struct Impl;
  - private:
  std::unique_ptr<Impl> impl_;
  TF_DISALLOW_COPY_AND_ASSIGN(ThreadPool);
  };
  