The background compilation logic moved to the {CompilationState}, so
the {NumberOfBackgroundTasks} is now always 1.

R=mstarzinger@chromium.org

Bug: chromium:869420
Change-Id: I5860f46db953778890c842af4507e8d989e34784
Reviewed-on: https://chromium-review.googlesource.com/1158581
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54847}