Use ScopedTaskEnvironment instead of MessageLoop in ios/chrome/app/application_delegate

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.

This is a reland of https://crrev.com/c/1649167 but not initializing the
ThreadPool to prevent the tests from crashing.

BUG=891670
BUG=972851

Change-Id: Ie7e9ef44562ff5a6613b480f39b8e05ec127db58
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1651812
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#668341}