This flag will be useful to analyze (and then improve) fragmentation
and compaction.

Bug: v8:9329
Change-Id: Ie3c5b618fa5717277cf2b9617e2d8f9dc2138056
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1664333Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Darius Mercadier <dmercadier@google.com>
Cr-Commit-Position: refs/heads/master@{#62285}