This reverts commit 231b3e15.

Reason for revert: Flakes in inspector/debugger/wasm-stack

Original change's description:
> [wasm] Move code generation fully to background task.
> 
> This moves the entire code generation phase (including code emission
> into the native module) into the background task. The code manager is
> fully thread safe by now and there are no Isolate-bound steps anymore.
> 
> R=​clemensh@chromium.org
> BUG=v8:7921
> 
> Change-Id: Ie3e8565e126bfdb58bf472291a1f9fbebe7b5431
> Reviewed-on: https://chromium-review.googlesource.com/1128743
> Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
> Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#54324}

TBR=mstarzinger@chromium.org,clemensh@chromium.org

Change-Id: Ice5d80425cebd3921d9683d06f70d5173f663e42
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:7921
Reviewed-on: https://chromium-review.googlesource.com/1129059Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54326}