Use {std::unique_ptr} instead of raw {malloc}ed memory. This increases
readability and memory safety significantly. The only downside is that
we cannot use {realloc} any more (there is no C++ replacement for that),
but that should not be noticeable.

Drive-by: Make fields private.

R=mstarzinger@chromium.org

Bug: v8:10021
Change-Id: Ie99d61fe136fc18a07aa45f38a5a9b6542308504
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1940261Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65234}