For problems with the current approach, see crbug.com/v8/7564.

We can instead gather all weak references in code in VisitEmbeddedPointer.

BUG=v8:7564, v8:7308

Change-Id: Ib369e7ab9efd62c90bdac69835318929c58217f2
Reviewed-on: https://chromium-review.googlesource.com/968250
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52041}