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}