[scheduling] Use ScopedTaskEnvironment instead of MessageLoop in app_service

MessageLoop will go away, eventually.

BUG=891670

Change-Id: Idb748b2f95c82c89e0e4a2a6846fb6221cdf5586
Reviewed-on: https://chromium-review.googlesource.com/c/1451884
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#629086}
1 file changed