This reverts commit 9d0154d6.

Reason for revert: gc stress breakage, prob this commit is broken.

Original change's description:
> [heap] Verify FeedbackVector contents during heap verification.
> 
> Pre-work for in-place weak refs.
> 
> BUG=v8:7308
> 
> Change-Id: I5c7086bded14879f62ab366d0aba59302f0078d3
> Reviewed-on: https://chromium-review.googlesource.com/1010069
> Commit-Queue: Marja Hölttä <marja@chromium.org>
> Reviewed-by: Igor Sheludko <ishell@chromium.org>
> Reviewed-by: Michael Stanton <mvstanton@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#52681}

TBR=marja@chromium.org,yangguo@chromium.org,mvstanton@chromium.org,ishell@chromium.org

Change-Id: I7d95e375e06db186f0e9823cb080ce15d1e1062d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:7308
Reviewed-on: https://chromium-review.googlesource.com/1019180Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52682}