The time spent by the parallel scavengers running on the main thread
was being added twice to the SCAVENGER_SCAVENGE_PARALLEL scope.

Change-Id: I358b28cbf56f554d04e3da927182a7c1a7568dad
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3253341
Commit-Queue: Nikolaos Papaspyrou <nikolaos@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77617}