This CL shares most of the implementation in the string
builtins String.prototype.charAt/charCodeAt/codePointAt.

Bug: v8:7270
Change-Id: Ibe43a0a22aa17fb5cd7f0519fd877fa8ae483863
Reviewed-on: https://chromium-review.googlesource.com/861786
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50546}