The single argument version of String.fromCharCode had an unneccessary
runtime call in it; this CL removes it.

Bug: v8:7531
Change-Id: Ieeb7d8aaffb0eabf814a52c51eaff50cb57d87c9
Reviewed-on: https://chromium-review.googlesource.com/955646
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51870}