Simplifies the unoptimized compile job to have only three steps, the
on-main-thread prepare step, the off-thread compile step and the
on-main-thread finalization step.

As part of this change, the compiler dispatcher no longer supports
functions with outer scopeinfo's, since these need to be analysed on the
main thread.

BUG=v8:5203

Change-Id: Ifb378ef81bd47b6f6d4037a3b8acf88660896c4e
Reviewed-on: https://chromium-review.googlesource.com/774558
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49413}