Untangles the non-tracing GC optimization (Scavenger) that allows for
dropping objects that are only reachable from certain API references
from EmbedderHeapTracer. Instead, allow setting it on Isolate.

This allows for using the optimization when using cppgc.

Chromium-side: https://crrev.com/c/2844587

Bug: chromium:1056170
Change-Id: I20f28dd84c808872c7f9559c8c168e828794dd1d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2844657Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74135}