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 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