Add v8::CppHeap as an implementation of a cppgc heap that
integrates with V8's existing EmbedderHeapTracer API. The
current implementation only supports non-incremental marking.

Bug: chromium:1056170
Change-Id: I4a09eb5ae57f5c7defe35eb3fe346627eb492473
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2245610
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68374}