Adds override annotation to FreeList subclass destructors.

Change-Id: I86034d381a408abe6e6c91ffb062999c0614799d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2161071
Commit-Queue: Dan Elphick <delphick@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67316}