This open-label, multicenter study will evaluate the safety, tolerability, and pharmacokinetics of MEHD7945A in combination with chemotherapy (either cisplatin plus 5-FU or carboplatin plus paclitaxel) in participants with previously untreated R/M SCCHN. There are two stages for each arm in this study: a Dose-limiting Toxicity (DLT)-evaluation stage (Stage I) and a cohort-expansion stage (Stage II). In Stage I, DLTs will be assessed during a DLT Assessment Window of 21 days (i.e., Cycle 1 Day 1 through Cycle 1 Day 21) for both arms. In Stage II, participants will be enrolled to further characterize the safety, pharmacokinetics, and anti-tumor activity of MEHD7945A in combination with cisplatin + 5-FU or carboplatin + paclitaxel at the identified recommended Phase II dose.