Now we can remove FixedArrayOfWeakCells (this was the last user).

Previous try: https://chromium-review.googlesource.com/1150170

BUG=v8:7308

Change-Id: Ie924e379ea8bbd797430e3ca591019fe001e78ad
Reviewed-on: https://chromium-review.googlesource.com/1154909Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54928}