Refactors the CompilerDispatcher to be able to enqueue eager inner functions
for off-thread compilation during top-level compilation of a script.

Unoptimized compile jobs are simplified to only have two phases - compile
and finalization. Only finalization requires heap access (and therefore
needs to be run on the main thread). The change also introduces a requirement
to register a SFI with a given compile job after that job is posted, this
is due to the fact that an SFI won't necessarily exist at the point the job
is posted, but is created later when top-level compile is being finalized.
Logic in the compile dispatcher is update to deal with the fact that a job
may not be able to progress if it doesn't yet have an associated SFI
registered with it.

BUG=v8:8041

Change-Id: I66cccd626136738304a7cab0e501fc65cf342514
Reviewed-on: https://chromium-review.googlesource.com/1215782
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56088}