Now we can finally get rid of Map::weak_cell_cache!

BUG=v8:7308

Change-Id: I87a06509bf638bf6833ea2ba1eca525fb4b15df1
Reviewed-on: https://chromium-review.googlesource.com/1128882
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54511}