Oslo’s revitalized waterfront shines with modern architectural drama. The National Opera house beckons visitors to stroll across its sloping marble roof. Brooding masterpieces evoke contemplation inside the angular Munch Museum. Electric tour boats glide across the azure water of the Oslofjord.   Yet just steps from the harbor stands a quieter landmark, a century-old building with a tile roof that links Norway and North America.   Thousands of emigrants once began their transatlantic journeys here, departing from the headquarters of the Norwegian America Line. Clutching...