Reland of https://chromium-review.googlesource.com/c/558290/

Makes compiler dispatcher jobs an abstract interface, with unoptimized
compile jobs as an implementation of this interface.

Bug: v8:6537
Change-Id: Ia85781f72c7aaca497896ca4efa91ada97e43b1c
Reviewed-on: https://chromium-review.googlesource.com/589154
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46940}