Replaces ItemParallelJob by std::vector to hold marking items.
IndexGenerator is used to iterate over evacuation items.
slots_ is moved from items to YoungGenerationMarkingTask to reduce
synchronisation.

Change-Id: Iac7aba215e8ba545c12a9ab6c810d343234fbbbf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2440830
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70575}