The DefineDataProperty in JSStructuredCloneReader::readObjectField is pretty slow, see the profile in bug 1811666. We can get rid of most of these calls with a fast path for plain object properties and dense array elements.

This improves the micro-benchmark below from ~1200 ms to ~930 ms.

function f() {
    var arr = [];
    for (var i = 0; i < 50; i++) {
        arr.push(i, {x: 1});
    }
    var buf = serialize(arr);
    var t = new Date;
    var res;
    for (var i = 0; i < 100_000; i++) {
        res = deserialize(buf);
    }
    print(new Date - t);
    return res;
}
f();

On typical workloads, these fast paths for plain objects and array objects handle
at least 97% of cases. This is much faster than the generic DefineDataProperty code.

https://hg.mozilla.org/mozilla-central/rev/b9c4ba784620

Looking at the deserialize_us telemetry probe, after this landed the 95% percentile improved consistently from 72 to 60 (-16.7%). The 50% percentile from 3 to 2 (-33%).