A Study of Paclitaxel With GDC-0941 Versus Paclitaxel With Placebo in Participants With Locally Recurrent or Metastatic Breast Cancer

Inclusion Criteria: * Histologically or cytologically confirmed adenocarcinoma of the breast, with measurable or non-measurable locally recurrent or metastatic disease * Human epidermal growth factor receptor 2 (HER2)-negative and hormone receptor (HR) (estrogen receptor and/or progesterone receptor)-positive disease as defined by local guidelines * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 * Adequate hematologic and end organ function * Women of childbearing potential must agree to remain abstinent or to use two adequate methods of contraception, including at least one method with a failure rate of less than (\<) 1 percent (%) per year, during the treatment period and for at least 30 days after the last dose of study treatment or 6 months after discontinuation of paclitaxel, whichever is longer Exclusion Criteria: * Prior non-capecitabine chemotherapy for locally recurrent or metastatic disease * Prior treatment with a phosphoinositide 3-kinase (PI3K) inhibitor for advanced or metastatic breast cancer * History of intolerance to a taxane-containing therapy * History of clinically significant cardiac or pulmonary dysfunction * History of malabsorption syndrome or other condition that would interfere with enteral absorption * Clinically significant history of liver disease * Active autoimmune disease or active inflammatory disease * Immunocompromised status due to current known active infection with human immunodeficiency virus (HIV) or due to the use of immunosuppressive therapies for other conditions * Need for current chronic corticosteroid therapy * Pregnant, lactating, or breastfeeding women * Current severe, uncontrolled systemic disease * Known untreated or active central nervous system (CNS) metastases