If you are found to be eligible to take part in this study, you will be enrolled in 1 of 2 study groups: Group A or Group B. Both groups will receive the same drugs at the same schedule, and the same dose of atezolizumab and eribulin. However, Group A may receive a different dose of cobimetinib. Up to 9 participants will be in Group A and 24 participants will be in Group B.