... So that this can be re-used in the intl code. Also, add a helper
method that loads the iterator if it's not provided.

Bug: v8:5751
Change-Id: Ifc1142a486b2cdf1c33a813c5446b206ad9f81ef
Reviewed-on: https://chromium-review.googlesource.com/1156390
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54819}