The NFL is a TV ratings juggernaut: 19 of the 20 most-viewed broadcasts in U.S. history are Super Bowls. But the league is facing new challenges to its dominance in the age of mobile video, at the same time as America's Game has become a cultural flashpoint. The NFL's head of digital, Maryann Turcke, explains how she dealt with player protests, covered off-the-field controversies, and tried to keep the focus on football during a tumultuous year for the league.