CppHeap was missing a scope for incremental marking.

This CL also introduces NestedEmbedderStepScope which is used for
identifying nested samples to avoid double accounting in UMA.

Bug: chromium:1056170
Change-Id: I8bba3fbfe6d098fe6861d1cfe5df8b88b4ac0fea
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2642260
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72265}