The foreground task runner and the background task runner are the same.
Thereby we can get predictable behavior.

R=clemensh@chromium.org

Change-Id: I18f9c7277a344b7884d6de0c2159cc3f010576b4
Reviewed-on: https://chromium-review.googlesource.com/771833Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49383}