This reverts commit 8fa7f9ed.

Reason for revert: Speculating that this breaks GC stress

Original change's description:
> [runtime] Improve for-in performance
> 
> - Add fast-path String conversion for Smi (which is the most common case)
>   This improves for-in by ~10% on non-initialized enum-caches
> - Don't use the NumberStringCache for large indices to not overflow the cache
>   during key collection. This improves worst-case performance by ~2.5x
> - Drop number_to_string_native and number_to_string_runtime counters
> 
> Bug: v8:7717
> Change-Id: Ic1ff385e3374e6a7e7e7bdb9ae75fb8c238105d1
> Reviewed-on: https://chromium-review.googlesource.com/1167049
> Reviewed-by: Toon Verwaest <verwaest@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Commit-Queue: Camillo Bruni <cbruni@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#55233}

TBR=ulan@chromium.org,cbruni@chromium.org,verwaest@chromium.org

Change-Id: I8d0332478afcd7c6a3f8fbf1f044b9aa870b6b13
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:7717
Reviewed-on: https://chromium-review.googlesource.com/1182676Reviewed-by: Aseem Garg <aseemgarg@chromium.org>
Commit-Queue: Aseem Garg <aseemgarg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55241}