Previously StringToList use the length of the original string, which is
not the right value: we expect the length of the new array to be the
number of characters (codepoints).

Bug: v8:7980
Change-Id: I2efca5715323c4399cb45c53871ae349207f3458
Reviewed-on: https://chromium-review.googlesource.com/c/1297320
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56944}