An unnecessary call to ToString() on the array index caused trips to
the runtime. The fix also includes performance micro-benchmarks so
we'll have a harder time regressing this case in future.

Bug: v8:8112
Change-Id: Iada5bd2e3c6d2246fb1225e7094f3d9c66ddafbd
Reviewed-on: https://chromium-review.googlesource.com/1206355
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55653}