- Create items for each page containing OLD_TO_NEW references.
- Introduce the flags for parallel scavenge since this forces
  us to introduce Tasks.

Bug: chromium:738865
Change-Id: Idad63f4318bdb3786117441e5413eb5e8594b7fb
Reviewed-on: https://chromium-review.googlesource.com/575052
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46748}