message: Fixed small nits in WhitelistedStatefulOpRegistry
  StringPiece has been changed to string to avoid static order destruction fiasco (we store pointers that might have shorter lifetime) and also to use unordered_set (there is hash specialization for StringPiece).
diff: @@ -16,6 +16,7 @@ limitations under the License.
  #ifndef TENSORFLOW_CORE_FRAMEWORK_DATASET_STATEFUL_OP_WHITELIST_H_
  #define TENSORFLOW_CORE_FRAMEWORK_DATASET_STATEFUL_OP_WHITELIST_H_
  +#include <unordered_set>
  #include "tensorflow/core/lib/core/status.h"
  namespace tensorflow {
  @@ -24,27 +25,26 @@ namespace data {
  // See below macro for usage details.
  class WhitelistedStatefulOpRegistry {
  public:
  - Status Add(StringPiece op_name) {
  - op_names_.insert(op_name);
  + Status Add(string op_name) {
  + op_names_.insert(std::move(op_name));
  return Status::OK();
  }
  - bool Contains(StringPiece op_name) {
  - return op_names_.find(op_name) != op_names_.end();
  - }
  + bool Contains(const string& op_name) { return op_names_.count(op_name); }
  static WhitelistedStatefulOpRegistry* Global() {
  - static WhitelistedStatefulOpRegistry* reg =
  - new WhitelistedStatefulOpRegistry;
  + static auto* reg = new WhitelistedStatefulOpRegistry;
  return reg;
  }
  private:
  - WhitelistedStatefulOpRegistry() {}
  - WhitelistedStatefulOpRegistry(WhitelistedStatefulOpRegistry const& copy);
  + WhitelistedStatefulOpRegistry() = default;
  + WhitelistedStatefulOpRegistry(WhitelistedStatefulOpRegistry const& copy) =
  + delete;
  WhitelistedStatefulOpRegistry operator=(
  - WhitelistedStatefulOpRegistry const& copy);
  - std::set<StringPiece> op_names_;
  + WhitelistedStatefulOpRegistry const& copy) = delete;
  +
  + std::unordered_set<string> op_names_;
  };
  } // namespace data
  