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.

TBR=tebbi@chromium.org

Bug: v8:8112
Change-Id: I781e8b1bbe2eb56db961cf33b0dca8523868b83d
Reviewed-on: https://chromium-review.googlesource.com/1213207
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55733}