When the main thread contributes to an item parallel job and runs
an item parallel task, it currently emits a background GC trace event.

That is confusing and may lead to incorrect accounting of main thread
GC time. This patch fixes it by introducing a 'Runner' parameter
to ItemParalllelJob::Task::RunInParallel and emitting a foreground
GC event if the runner is the main thread.

Bug: v8:9508
Change-Id: I755751bfe9eef427666d5f16fb50aa6093059e80
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1706485Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62798}