Enables following JS references for unified heap snapshots. Any object
that's referencing a JS objects is marked as visible.

Followup:
- Handling (merging) of wrapper/wrappable pairs.

Change-Id: I02d41a3224265f38d934dcb2686ac24b49c1dbd7
Bug: chromium:1056170
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2489698Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70735}