AllocationCounter just stores all allocation observers for now.

Bug: v8:10315
Change-Id: I4ff4208877dd9454c9eef5e5d2e2349b7f00065f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2306793
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68942}