This study is being conducted to determine the efficacy and safety in using Atezolizumab (MPDL3280A) in combination with nab-paclitaxel in treating patients with metastatic breast cancer. The safety of single-agent nab-paclitaxel has been determined in previous studies of patients with metastatic breast cancer, and preliminary data shows the use of MPDL3280A is safe to use in combination with chemotherapy.