This reverts commit f3c8da56.

Reason for revert: GC stress failures
https://build.chromium.org/p/client.v8/builders/V8%20Linux64%20GC%20Stress%20-%20custom%20snapshot/builds/15396

Original change's description:
> [heap] Use weak cell in normalized map cache.
> 
> This replaces ad-hoc weakness in normalized map cache with weak cell.
> 
> Bug: chromium:694255
> Change-Id: I6a12301b2176fe3723b56178a65582cfb412f7d2
> Reviewed-on: https://chromium-review.googlesource.com/704834
> Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#48344}

TBR=ulan@chromium.org,mlippautz@chromium.org

Change-Id: I0b2d39a1dcff6416998ab36506ee950220c87e89
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:694255
Reviewed-on: https://chromium-review.googlesource.com/705194Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48349}