On the PredictablePlatform we return the ForegroundTaskRunner of the
underlying platform in both GetForeGroundTaskRunner and
GetBackgroundTaskRunner. The reason is that thereby we can enforce a
predictable, sequential execution of tasks.

R=clemensh@chromium.org, rmcilroy@chromium.org

Change-Id: Icec9fe52da922b1e75a3fb5b0155083be0a3a0fd
Reviewed-on: https://chromium-review.googlesource.com/771792Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49384}