A Study of Carboplatin Plus Etoposide With or Without Atezolizumab in Participants With Untreated Extensive-Stage (ES) Small Cell Lung Cancer (SCLC)

Inclusion Criteria: * Histologically or cytologically confirmed ES-SCLC (per the Veterans Administration Lung Study Group \[VALG\] staging system) * No prior systemic treatment for ES-SCLC * Eastern Cooperative Oncology Group performance status of 0 or 1 * Measurable disease, as defined by RECIST v1.1 * Adequate hematologic and end organ function * Treatment-free for at least 6 months since last chemo/radiotherapy, among those treated (with curative intent) with prior chemo/radiotherapy for limited-stage SCLC Exclusion Criteria: * Active or untreated central nervous system (CNS) metastases as determined by computed tomography (CT) or magnetic resonance imaging (MRI) evaluation * Malignancies other than SCLC within 5 years prior to randomization, with the exception of those with a negligible risk of metastasis or death treated with expected curative outcome * Pregnant or lactating women * History of autoimmune disease * History of idiopathic pulmonary fibrosis, organizing pneumonia, drug-induced pneumonitis, idiopathic pneumonitis, or evidence of active pneumonitis on screening chest CT scan. History of radiation pneumonitis in the radiation field (fibrosis) is permitted. * Positive test result for human immunodeficiency virus (HIV) * Active hepatitis B or hepatitis C * Severe infections at the time of randomization * Significant cardiovascular disease * Prior treatment with cluster of differentiation (CD) 137 agonists or immune checkpoint blockade therapies, anti-programmed death-1 (PD-1), and anti-PD-L1 therapeutic antibody * History of severe (or known) hypersensitivity to chimeric or humanized antibodies or fusion proteins or any component of atezolizumab formulation.