The dramatic reshaping of Zillow’s place in the real estate ecosystem is about to take a giant step forward, as the online real estate behemoth announced Monday that it is getting into the mortgage business. Nearly 18 months ago, Zillow shook up the real estate industry when it announced that it was getting into the home selling business by launching “Zillow Instant Offers.” In that program, prospective home sellers in certain markets can receive offers for their home direct from investors, all through Zillow’s platform.