Revert of Replace ad-hoc weakness in transition array with WeakCell. (patchset #5 id:80001 of https://codereview.chromium.org/1157943003/)

Reason for revert:
Breaks descriptor array clearing.

Original issue's description:
> Replace ad-hoc weakness in transition array with WeakCell.
>
> BUG=
>
> Committed: https://crrev.com/885455e99de817f86a0b5df2dc0d932cfc179749
> Cr-Commit-Position: refs/heads/master@{#29083}

TBR=jkummerow@chromium.org,hpayer@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/1194673002

Cr-Commit-Position: refs/heads/master@{#29121}