A Study of Trastuzumab Emtansine in Combination With Atezolizumab or Placebo as a Treatment for Participants With Human Epidermal Growth Factor 2 (HER2)-Positive and Programmed Death-ligand 1 (PD-L1)-Positive Locally Advanced (LABC) or Metastatic Breast Cancer (MBC)

Inclusion Criteria: * HER2+ and PD-L1+ locally advanced (LABC) or metastatic breast cancer (MBC) * Progression must have occurred during most recent treatment for LABC/MBC or during, or within 6 months after completing, neoadjuvant and/or adjuvant therapy * Prior treatment with trastuzumab (+/- pertuzumab) and taxane in the neoadjuvant and/or adjuvant, locally advanced, or metastatic setting * No more than two prior lines of therapy in the metastatic setting * Measurable disease per RESIST version 1.1 * Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1 * Life expectancy \>= 6 months * Adequate hematologic and end-organ function * For women of childbearing potential: agreement to remain abstinent or use contraception, and agreement to refrain from donating eggs * For men: agreement to remain abstinent or use contraceptive measures, and agreement to refrain from donating sperm Exclusion Criteria: * Prior treatment with trastuzumab emtansine in metastatic setting * History of exposure to cumulative doses of anthracyclines * Symptomatic or actively progressing central nervous system (CNS) metastases; asymptomatic CNS lesions ≤ 2cm without clinical requirement for local intervention or asymptomatic patients with treated CNS lesions are eligible * Current Grade \>= 3 peripheral neuropathy * Cardiopulmonary dysfunction * History of malignancy within 5 years prior to initiation of study treatment, with the exception of the cancer under investigation and malign…