- Store dirty JSWeakFactories in a heap root (not native context) -
during GC there's no native context necessarily.

- Schedule one microtask per JSWeakFactory.

- Enter the context of the cleanup function before calling it.

BUG=v8:8179

Change-Id: Icaa245a08a60dd7325af828858ebe55d842c5bf6
Reviewed-on: https://chromium-review.googlesource.com/c/1298899
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57081}