cppgc must support the same feature set as the existing unified heap
system, which requires support for wrapper-specific handling (drop on
Scavenge, merge in snapshot).

Replace JSMember by TracedReference to support IsRootForNonTracingGC()
optimizations out of the box. cppgc support for wrapper/wrappable
pairs will be added as followup.

Change-Id: I3c6eff2b8dce5b71b04b2bd75182eb8672079a64
Bug: chromium:1056170
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2498685
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70801}