We assume that at this point the platform always exists. If this
assumption fails we have to reconsider how we call foreground tasks from
background tasks.

R=clemensh@chromium.org

Bug: chromium:764313
Change-Id: Ic2e61adc138cdf969f5b0bdf7702e839df5846b9
Reviewed-on: https://chromium-review.googlesource.com/663717Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47983}