Keep tasks in unique_ptrs, such that they are freed independent of
whether they have been executed or not.

R=szuend@chromium.org

Bug: chromium:1142437, v8:11107, v8:11074
Change-Id: Ia265df3187c724b63e0f576d33235c1bfa522c4f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2517694
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71005}