Change ListFormat.prototype.formatToParts to use FieldPositionIterator

Bug: v8:8152
Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: I03e64f9843e55e624fa9ec090f5b3db7b4949f47
Reviewed-on: https://chromium-review.googlesource.com/c/1215273
Commit-Queue: Frank Tang <ftang@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57199}