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

Change-Id: Id687f6696e74998c9d23ee2a2ee97c7687d13815
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2438631
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70263}