Marking on allocation was missing the top level scope.
Also adding a dedicated scope for on allocation to more clearly
distinguish it in traces.

Bug: chromium:1056170
Change-Id: I1b7d80c9f171f81988826de0174ef5b00d6f1d34
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2891572
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Auto-Submit: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74520}