The method allocates, so it should return the buffer in a {unique_ptr}.
Also, the internals can be simplified by using {size_t} instead of
{int} and removing a redundant special case.

R=mlippautz@chromium.org

Bug: v8:9810
Change-Id: I94ac5814c284bf6ab075841ddbfb768d31dfff4c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1849514
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64195}