The four standard chemotherapy choices were carboplatin + paclitaxel, paclitaxel alone, dacarbazine, or temozolomide. Participants on standard chemotherapy who experienced disease progression may have been eligible to crossover to treatment with pembrolizumab provided they met protocol-specified requirements for crossover. With Amendment 03, all crossover participants will be treated with open-label, fixed dose pembrolizumab 200 mg Q3W instead of weight-based dosing of pembrolizumab.