Use ScopedTaskEnvironment instead of MessageLoop in /ui/gfx/image

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=rsesek@chromium.org

Change-Id: Ia002715c01ed6fbd2e7973068a33f64a07cf7a70
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1649548
Auto-Submit: Carlos Caballero <carlscab@google.com>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678733}
2 files changed