SANTA CLARA, Calif.—Kyle Shanahan was a teenager watching on the sideline when the heated playoff rivalry between the Dallas Cowboys and San Francisco 49ers reached a fever pitch with three straight NFC title game matchups. Now Shanahan will take a starring role when one of the NFL’s great rivalries resumes as head coach of the 49ers instead of son of their offensive coordinator when San Francisco visits Dallas on Sunday in a wild-card matchup. “I think that’s as cool as it gets because that’s the coolest part of my childhood growing up I feel like,” Shanahan said. “It was seventh grade, eighth grade, ninth grade, ’92, ’93, ’94, every single year. I thought we were the best team in the league … I remember being on that sideline and just watching how good the Cowboys were and it was unbelievable. And the next year, going to Dallas for the home-field …