MO42319 KATE3: A randomized, multicenter, double-blind, placebo-controlled phase III study of the efficacy and safety of trastuzumab emtansine in combination with atezolizumab or placebo in patients with HER2-positive and PD-L1- positive locally advanced or metastatic breast cancer who have received prior trastuzumab- (+/- pertuzumab) and taxane-based therapy (kate3)