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)

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 malignancies with a negligible risk of metastasis or death
• •History of leptomeningeal disease
• •Active or history of autoimmune disease or immune deficiency
• •Active hepatitis B, hepatitis C and/or tuberculosis
• •Prior allogeneic stem cell or solid organ transplantation
• •Receipt of a live, attenuated vaccine within 4 weeks prior to initiation of study treatment, during treatment, or within 5 months following the last dose of study treatment
• •Pregnancy or lactation