This is resurrecting an optimization from the pre-torque version.
Avoid allocating another sequential string for the result when there's only one
element, just return the ToString-ed element.  This not only saves time writing
to this destination string, but also reduce GC pressure.

The System Health Memory Benchmark (load:media:google_images) exposed this missing
optimization with a 15% regression in memory usage.  Very large external strings
were being copied into V8's heap as sequential string.

Bug: chromium:896612
Change-Id: Ieb61906f64100cdc15bf96f3ebcccb1207f75356
Reviewed-on: https://chromium-review.googlesource.com/c/1316620Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Peter Wong <peter.wm.wong@gmail.com>
Cr-Commit-Position: refs/heads/master@{#57241}