Phase Ib Dose-escalation Trial of Taselisib (GDC-0032) in Combination With Anti-HER2 Therapies in Participants With Advanced HER2+ Breast Cancer

This study is divided in two parts, a combination dose finding escalation part (Part 1) and a dose combination expansion part (Part 2). Participants will enter only one Part (either 1 or 2) and receive study drugs from only one combindation of study drugs, know as arms, as assigned by the main study physician. The study includes four different arms as listed below: * Arm A: Taselisib with Trastuzumab emtansine (also called T-DM1) * Arm B: Taselisib with Trastuzumab emtansine and Pertuzumab * Arm C: Taselisib with Pertuzumab and Trastuzumab * Arm D: Taselisib with Pertuzumab, Trastuzumab, and Paclitaxel Part 1: Since we are looking for the highest dose of Taselisib that can be administered safely without severe or unmanageable side effects in participants that have breast cancer, not everyone who participates in Part 1 of this research study will receive the same dose of the Taselisib. The dose participants get will depend on the number of participants who have been enrolled in the study before and how well they have tolerated their doses. Each combination dose will only be given to a group of 3 - 6 participants. The results from each group will be reviewed and depending on the results, a different combination dose or schedule may be investigated in the next group of participants or the same combination dose taken by a participant may be repeated with the next group of participants to investigate these results further (a different schedule means that instead of taking doses once every day, participants may take them only on some days in the week). Part 2: The doses in this part will be based on the best combination doses from Part 1. This part will look at the potential side effects and see how your cancer responds to the drug.