Use ScopedTaskEnvironment instead of MessageLoop

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=bajones@chromium.org

Change-Id: Icf167d4cc8395db1bad4cc2252b4acb8bbb5c691
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1635613Reviewed-by: Brandon Jones <bajones@chromium.org>
Commit-Queue: Brandon Jones <bajones@chromium.org>
Auto-Submit: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#664839}

services/device/public/cpp/power_monitor/power_monitor_broadcast_source_unittest.cc

@@ -5,9 +5,9 @@
 #include "services/device/public/cpp/power_monitor/power_monitor_broadcast_source.h"
 
 #include "base/macros.h"
-#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/test/power_monitor_test_base.h"
+#include "base/test/scoped_task_environment.h"
 #include "base/threading/sequenced_task_runner_handle.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ -38,7 +38,7 @@ class PowerMonitorBroadcastSourceTest : public testing::Test {
 
   base::PowerMonitor* monitor() { return power_monitor_.get(); }
 
-  base::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
 
  private:
   PowerMonitorBroadcastSource* power_monitor_source_ptr_;