- 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/1167049Reviewed-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}