We set the JSArray's length to the elements's length before,
which is wrong when the elements have the slack part. We could
serialize the correct length and only the actual elements excluding
the slack part for JSArray's elements. And we do the same thing
for the objects to avoid serializing unnecessary elements.

Bug: v8:13304
Change-Id: Ib68e06f409bfcab5c57fb5532e188aa0099d1140
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3905061Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83380}