Use ScopedTaskEnvironment instead of MessageLoop in /ppapi

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=raymes@chromium.org

Change-Id: Ib494b6fb9c77e44fa27c9afb2b98c4f0b775b892
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1648208
Auto-Submit: Carlos Caballero <carlscab@google.com>
Reviewed-by: Raymes Khoury <raymes@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Cr-Original-Commit-Position: refs/heads/master@{#667946}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 7de7f44fdd1e1782744948daeeaae22f35fe439f
4 files changed