This CL adds a regression benchmark for a fast-path of
String.p.charCodeAt, which is important for node.js.

Bug: v8:7326
Change-Id: I54efaa2988c595dd40e6a55a3464b3ee7de6f07b
Reviewed-on: https://chromium-review.googlesource.com/942885Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51654}