message: Make the VerifiedJwt constructor private.
  As discussed offline, users should only get VerifiedJwt objects from the Verify resp Mac functions.
diff: @@ -41,8 +41,6 @@ class JwtMacImpl;
  // information (typ, cty, alg and kid).
  class VerifiedJwt {
  public:
  - VerifiedJwt();
  -
  // VerifiedJwt objects are copiable and movable.
  VerifiedJwt(const VerifiedJwt&) = default;
  VerifiedJwt& operator=(const VerifiedJwt&) = default;
  @@ -80,6 +78,7 @@ class VerifiedJwt {
  util::StatusOr<std::string> ToString();
  private:
  + VerifiedJwt();
  explicit VerifiedJwt(const RawJwt& raw_jwt);
  friend class jwt_internal::JwtMacImpl;
  RawJwt raw_jwt_;
  