Replaces ItemParallelJob by std::vector to hold work items.
IndexGenerator is used to iterate over evacuation items.

Change-Id: I63ea246f267d8cbe140c47c022b95b3873bc957a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2425339
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70242}