Use ScopedTaskEnvironment instead of MessageLoop in /ppapi/proxy

MessageLoop will go away, eventually.

ScopedTaskEnvironment will per default start a ThreadPool, which should
be fine in most of the cases. If you belive your test needs to make sure
that no ThreadPool runs let me know and I will update the patch.

BUG=891670
This CL was uploaded by git cl split.

R=piman@chromium.org

Change-Id: Ida93c455e32bf058063b6bf48800b289c972823b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1649326
Auto-Submit: Carlos Caballero <carlscab@google.com>
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Antoine Labour <piman@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#667191}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 87cbd6e88422ea70e50167d7113d2229b89e1497
2 files changed