Minimal implementation to run a simple example (see test) demonstrating the
weakness of WeakCell.

- Behind FLAG_harmony_weak_refs
- Add WeakFactory & WeakCell, no WeakRef in this version.

Spec clarifications: goo.gl/7ujBAk
Design doc: goo.gl/nvof2T

BUG=v8:8179

Change-Id: Iea2a7a2201e6380644235d190a542ab46e082382
Reviewed-on: https://chromium-review.googlesource.com/c/1238579Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56483}