This reverts commit c063e3f0.

Reason for revert: https://bugs.chromium.org/p/v8/issues/detail?id=7768

Original change's description:
> [in-place weak refs] Replace WeakCells in FeedbackVector.
> 
> BUG=v8:7308
> 
> Change-Id: I08b97f7f47e2d96e313b70a66cd890fedd46e9fb
> Reviewed-on: https://chromium-review.googlesource.com/1049606
> Commit-Queue: Marja Hölttä <marja@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Leszek Swirski <leszeks@chromium.org>
> Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
> Reviewed-by: Igor Sheludko <ishell@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#53233}

TBR=ulan@chromium.org,marja@chromium.org,leszeks@chromium.org,tebbi@chromium.org,ishell@chromium.org

Change-Id: I68c068883884f9311ff166306245d0f21693bb6f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:7308
Reviewed-on: https://chromium-review.googlesource.com/1065631Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53241}