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