This also fixes benchmark scores for String.prototype.indexOf

Bug: v8:7127, v8:7092
Change-Id: Iee0f9689feb5923b300e253c267a6f32ffd4da20
Reviewed-on: https://chromium-review.googlesource.com/846739Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50316}