Hendricks scored midway through the first period, and Seguin tied it in the second. It stayed that way through a scoreless third, with Washington killing off a penalty in the final 3 minutes to send the game into overtime.Patrice Bergeron had a chance to win it in the first minute of the extra period, but he couldn’t get off a solid shot from Holtby’s right. Two minutes later, the Capitals broke into the Boston zone with former Bruin Mike Knuble leading a 2-on-1.Knuble shot, and Thomas left the rebound out where Ward could reach it with his backhander.The building fell silent as the Capitals celebrated just their third postseason series win since a run to the Stanley Cup finals in 1998. Some fans littered the ice with debris, but the Bruins waited for the postgame handshake.Thomas, bringing up the rear, gave Holtby a tap on the shoulder and said, ‘‘Great job, kid.’’No team has repeated as Stanley Cup champions since the Detroit Red Wings in 1997-98.The Capitals earned more than 100 points in the previous three seasons, leading the NHL with 121 in 2010, but had little to show for it once the postseason started. They won just two series over that span and have not made it out of the Eastern Conference semifinals since 1998, when they made it to the Stanley Cup finals but were swept by Detroit.This year’s regular season wasn’t as successful.But the playoffs have a chance to be even better.Entering the postseason as a No. 7 seed, the Capitals won three times in Boston — they also won Games 2 and 5 — to earn a berth in the Eastern Conference semifinals. Their second-round opponent won’t be determined until after Game 7s Thursday night between Ottawa and the New York Rangers, and Florida and New Jersey.The Bruins needed an unprecedented three Game 7s to win the Cup last year, including the 4-0 victory over Vancouver that gave the Original Six franchise its first title since 1972. Thomas also had a shutout in the Game 7 win over Tampa Bay in the Eastern Conference finals.