Advertising executives who entered Radio City Music Hall on Monday for NBCUniversal’s upfront presentation were greeted by an orchestra playing the familiar themes of the network’s landmark shows, such as “Law & Order” and “NBC Nightly News.”It was a nod to broadcasting’s rich history, which for the rest of the week would be relegated to the distant past. Streaming video now makes up 37% of U.S. television viewing, better than either broadcast or cable TV, according to Nielsen data. The May ...