In predictable mode DoSync and DoAsync are only normal
function calls. Therefore I had to do some adjustments
to async compilation to make it work with --predictable:
* I moved all calls to DoSync and DoAsync out of
  DisallowHandleAllocation and DisallowHeapAllocation
  scopes.
* I turned off the use of the semaphore which
  synchronizes the background compilation tasks with
  the main thread. It caused a deadlock.
* Adjust when the AsyncCompileJob is deleted, namely
  after the start function and not after the execution
  of the last compilation task. The reason is that in
  predictable mode all previous tasks are still on the
  stack after the last compilation task.


Bug:

Change-Id: I2f96f64febeee6b8bd5f4da3cec882797d249400
Reviewed-on: https://chromium-review.googlesource.com/469610
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44456}