<qa>  What is the name of the cohort that will be in Cohort 3?This study will have 3 cohorts. Cohorts 1 and 2 will run simultaneously; Cohort 3 will start when biomarker assay validation is complete. In Cohort 1, participants who have received at least two prior therapies for their advanced disease will receive monotherapy with pembrolizumab. In Cohort 2, participants who have not received any previous therapy for their disease will receive pembrolizumab in combination with cisplatin and 5-FU or Japan only capecitabine. In Cohort 3, participants who have not received any previous therapy and who have programmed death ligand 1 PD-L1-positive tumors will receive pembrolizumab monotherapy. 