GlobalSize will be used as a hint to schedule scavenger work in
https://chromium-review.googlesource.com/c/v8/v8/+/2036661

This is implemented as an atomic variable that's updated when adding/removing
segments.

Bug: chromium:1012816
Change-Id: I8f6c3f10612f8febda9bfe640d91e235aa3c2f12
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2043273Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66175}