This reverts commit 8a27c7d3.

Reason for revert: 

Having more tasks then work items is intentional in some use cases, i.e. Scavenging where RunInParallel() does parallel processing on a dynamic workload *after* the initial set of work items:

    {
      barrier_->Start();
      TimedScope scope(&scavenging_time);
      PageScavengingItem* item = nullptr;
      while ((item = GetItem<PageScavengingItem>()) != nullptr) {
        item->Process(scavenger_);
        item->MarkFinished();
      }
      do {
        scavenger_->Process(barrier_);
      } while (!barrier_->Wait());
      scavenger_->Process();
    }

Original change's description:
> v8::ItemParallelJob : Do not launch more Tasks than there are Items to process.
> 
> Except when there are 0 items. For some reason I don't quite understand yet, not
> calling Run() on tasks_[0] when there are 0 items results in DCHECKs...
> 
> Bug: chromium:806237
> Change-Id: I38c8fffde64a42f93f4efda492832651137eebd7
> Reviewed-on: https://chromium-review.googlesource.com/888704
> Commit-Queue: Gabriel Charette <gab@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#50924}

TBR=gab@chromium.org,mlippautz@chromium.org

Change-Id: Iad2ab16bb41f339de8e3fbca1c08c5d26b8a0111
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:806237
Reviewed-on: https://chromium-review.googlesource.com/891186Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50928}