... for concurrent graph building.

Bug: v8:7790
Change-Id: I55eb419bda843670eff7de31e942dd8406c792f9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1682027
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62455}