I tried using HandDropChances and ArmorDropChances, after I discovered these are the new formats, and don't work. Here is my command:
summon Zombie ~ ~1 ~ {HandDropChances:[1.0f,1.0f],HandItems:[

,

]}
The HandItems does work, but the zombie won't drop the items, even after setting HandDropChances to 1.0 float (100% chance)