For a team filled with playoff newcomers, the Toronto Maple Leafs have grabbed the momentum in this series like a squad of veterans. Led by goaltender James Reimer, making his first appearance in NHL postseason play, the Leafs head into tonight’s seventh and deciding game against the Bruins with a lot of confidence. After a somewhat scrambly two periods at the Air Canada Centre last night, the young Leafs completely dominated the third, outhitting the Bruins and looking more like playoff-hardened veterans than stagestruck neophytes. Not bad for a team that was down 3-1. “Obviously, experience definitely counts for something in the playoffs,” said Leafs forward James van Riemsdyk, who assisted on both Toronto goals. “But . . . it’s a matter of making less mistakes and finishing your chances.” Van Riemsdyk, a relative playoff graybeard with 45 games to his credit, said teammates asked him about his experience with the Philadelphia Flyers when they overcame a 3-0 deficit against the Bruins three years ago. “I just told them you have to think about the next game and nothing else,” he said. “If you look at the big picture, it’ll look like a mountain.”