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/1128743Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54324}