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

BUG=v8:7308

Change-Id: I7b74f7833288b20d8f4d098a4afce640ea6db823
Reviewed-on: https://chromium-review.googlesource.com/1150170
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54776}