Adds a generic job that is based on items and tasks.

Bug: chromium:651354
Change-Id: I378e04741c5761ea6c4a74816b9af8ea22867f53
Reviewed-on: https://chromium-review.googlesource.com/506075
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45353}