Before this CL the C strings were generated using new char[] and
destroyed using delete. Fix this by avoiding a raw new[] call and
directly using unique_ptr.

Bug: chromium:1056170
Change-Id: I894171d58a3f7a765cdf86e07d3da2e9298a4a92
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2960275Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75147}