The primary objectives of this study are to determine 1) Progression-Free Survival (PFS) and 2) Overall Survival (OS) of pembrolizumab plus best supportive care (BSC) compared with placebo plus BSC. The primary hypotheses of this study are: 1) pembrolizumab plus BSC prolongs PFS per Response Evaluation Criteria in Solid Tumors (RECIST) v1.1, assessed by a blinded central imaging vendor compared to placebo plus BSC, and 2) pembrolizumab plus BSC improves OS compared with placebo plus BSC.