AssembleCode will eventually be moved into ExecuteJob, i.e., off
the main thread.

Bug: v8:6048
Change-Id: If84ee2aaca6c8827cb769c7d69e5094fb4f32e4b
Reviewed-on: https://chromium-review.googlesource.com/506669
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45372}