The web app owner who notified me about bugs v8:12112 and v8:12126 asked
me a reasonable question: "how am I ever supposed to trust the retaining
paths in the devtools, if the heap snapshot is generated by a different
component than the actual marking code?". This change is my attempt to
answer that question. If verification is enabled, the heap snapshot
generator will visit each heap object with a realistic marking visitor
to find all references from that object. It will then check that those
references match the HeapGraphEdges in the snapshot.

I also considered the idea that we could collect retaining information
during the last GC cycle before taking the heap snapshot, or during an
extra GC cycle immediately after. However, running the full GC provides
the embedder with the opportunity to run arbitrary code (including JS)
both before and after PerformGarbageCollection, so there is no clear
guarantee that the heap state during the snapshot actually matches the
heap state during marking.

Bug: v8:12112, v8:12126
Change-Id: Id29e75ecf9eee19e35daedbdb4a3e1df64785380
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3299590Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#78952}