This reverts commit e4bbf92b.

Reason for revert: Crashes: https://build.chromium.org/p/client.v8/builders/V8%20Linux64/builds/19156

Original change's description:
> [compiler-dispatcher] Make compiler jobs abstract
> 
> Makes compiler dispatcher jobs an abstract interface, with unoptimized
> compile jobs as an implementation of this interface.
> 
> Bug: v8:6537
> Change-Id: I6569060a89c92d35e4bc7962623f77082a354934
> Reviewed-on: https://chromium-review.googlesource.com/558290
> Reviewed-by: Georg Neis <neis@chromium.org>
> Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
> Commit-Queue: Leszek Swirski <leszeks@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#46931}

TBR=rmcilroy@chromium.org,neis@chromium.org,leszeks@chromium.org

Change-Id: I023c0455929180fdcde3caf581f483f794ca2368
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:6537
Reviewed-on: https://chromium-review.googlesource.com/589153Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46932}