We used to assume that we know the remaining entries returned by the
iterator based on the current index. However, that is not accurate,
since entries skipped by the current index could be deleted.

In the new approach, we allocate conservatively and shrink the result.

R=neis@chromium.org

Bug: v8:8433
Change-Id: I38a3004dc3af292daabb454bb76f38d65ef437e8
Reviewed-on: https://chromium-review.googlesource.com/c/1325966
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57360}