[scheduling] Use ScopedTaskEnvironment instead of MessageLoop in drive_backend

MessageLoop will go away, eventually.

BUG=891670

Change-Id: Ia64f78ea238b99b0599cfec331ffaa764742399b
Reviewed-on: https://chromium-review.googlesource.com/c/1377853
Reviewed-by: Taiju Tsuiki <tzik@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#623189}
3 files changed