Use ItemParallelJob to walk ephemerons in parallel.

Bug: chromium:844008
Change-Id: Iffc72422f7577458437764f42d13d3f2ee020758
Reviewed-on: https://chromium-review.googlesource.com/1100825
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53736}