perf: match ownKeys perf to the one of objectSpread #10189
This PR addresses a performance regression introduced at #10171.
In practice the most frequent use cases of objectSpread is merging two plain objects, which means we can skip a sweep of already enumerable property names. I also drafted a jsPerf snippet on the performance difference. It is 10% faster on V8 75 when merging two object with 512 keys.