Merges mask1 and mask2 into this FieldMask.

def Union(self, mask1, mask2):
    """Merges mask1 and mask2 into this FieldMask."""
    _CheckFieldMaskMessage(mask1)
    _CheckFieldMaskMessage(mask2)
    tree = _FieldMaskTree(mask1)
    tree.MergeFromFieldMask(mask2)
    tree.ToFieldMask(self)