This addresses the JSTests/Array/OptFastForEach regression,
which ends up spending a lot of time in the ToString builtin.

Bug: chromium:932919
Change-Id: I53cfdc61841bf10a669e54c3fdc009ead295782b
Reviewed-on: https://chromium-review.googlesource.com/c/1477068Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59664}