Cross-Over Phase: This is only applicable for participants randomized to receive SOC. Eligible participants will be treated with pembrolizumab for the remainder of the study or until disease progression, unacceptable AEs, intercurrent illness that prevents further administration of treatment, investigator's decision to withdraw the participant, noncompliance with study treatment or procedures requirements, the participant receives 35 treatments of study treatment (pembrolizumab arm only), or administrative reasons.