This avoids the need for the finisher task (running on the foreground
thread) for Liftoff code.
This CL just makes the simple change to call {AddCode} from the
background thread. More cleanup will follow in separate CLs.

R=mstarzinger@chromium.org

Bug: v8:6600, v8:7921
Change-Id: I99ef29377efee5be36ba203aa7ed71e2471d86f3
Reviewed-on: https://chromium-review.googlesource.com/1126930
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54266}