Fix dcheck failure where committed was smaller than used memory. This was because of background threads allocating between calculating both stats and used memory could already be larger due to those background allocations. Avoid this first calculating used memory and committed memory afterwards.

Bug: v8:10563, v8:10315
Change-Id: Ic07970f607941140e3028bddde3e365b66aa4b5f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2237138
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68256}