The Kronos Quartet, which revolutionized the chamber music experience and breathed life into a faltering new music movement when it formed back in the 1970s, turns 30 this year. Thirty years is a very long life for any quartet, let alone one which makes its living, as does Kronos, playing exclusively modern music. And while there are probably more daring ensembles about today, Kronos remains a revered original, and none of today’s successful contemporary music groups would deny that they owe much to the San Francisco-based foursome.