Boston is in the midst of a building boom, with thousands of new luxury units slated for construction over the next three years. But is the boom in high-end developments a bust for the city’s middle class? Mayor Marty Walsh says he is committed to balancing the ambitions of developers with the needs of struggling communities, a task that now falls on new BRA chief John Barros. He recently took WGBH News reporter Phillip Martin on a tour to explain how.