[scheduling] Use ScopedTaskEnvironment instead of MessageLoop in policy

MessageLoop will go away, eventually.

BUG=891670

Change-Id: If1187c1a952a492847b07a9cca210d77f140cca3
Reviewed-on: https://chromium-review.googlesource.com/c/1353886
Reviewed-by: Sergey Poromov <poromov@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#613480}
8 files changed