The StringEqual builtin handles two byte strings already.

Bug: v8:4913, v8:6365, v8:6371, v8:6936, v8:7022
Change-Id: I6f5a3999ccdce8e9bfcece6e94362c15183bbd8c
Reviewed-on: https://chromium-review.googlesource.com/778883Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49473}