This reverts commit c61f9171.

Reason for revert: (Speculative)
Seems to block the roll:
https://chromium-review.googlesource.com/c/chromium/src/+/753602

Also failures on webkit win unittests and gpu tests:
https://build.chromium.org/p/client.v8.fyi/builders/Win%20Release%20%28NVIDIA%29/builds/3382
https://build.chromium.org/p/client.v8.fyi/builders/V8-Blink%20Win/builds/11512

Original change's description:
> [compiler] Add background compilation mode.
> 
> Adds support for compiling top-level code on a background thread behind a flag.
> When the flag is enabled, any background-parsing-task will perform compilation
> as well as parsing.
> 
> BUG=v8:5203
> 
> Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
> Change-Id: I88ab05c97cd6aea8d6be26e27d8da327f2c9c3a8
> Reviewed-on: https://chromium-review.googlesource.com/741716
> Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
> Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
> Reviewed-by: Marja Hölttä <marja@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#49103}

TBR=rmcilroy@chromium.org,marja@chromium.org,mstarzinger@chromium.org

Change-Id: I49b0b0ee61fb79766a9a928b43d51d0eeb793d39
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:5203
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/753302Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49112}