No vessel owned by the United States or by any corporation in which the United States or its representatives shall own the entire outstanding capital stock or in the possession of the United States or of such corporation or operated by or for the United States or such corporation, and no cargo owned or possessed by the United States or by such corporation, shall after March 9, 1920, in view of the provision herein made for a libel in personam, be subject to arrest or seizure by judicial process in the United States or its possessions: Provided, That this chapter shall not apply to the Panama Canal Commission.