Function cef_string_map_append 

pub unsafe extern "C" fn cef_string_map_append(
    map: cef_string_map_t,
    key: *const cef_string_t,
    value: *const cef_string_t,
) -> c_int

Append a new key/value pair at the end of the string map. If the key exists, overwrite the existing value with a new value w/o changing the pair order.