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=sammiequon@chromium.org

Change-Id: I82d400ba9fa58b1532047c0781ea27f0da89ab44
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636351Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Auto-Submit: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#664788}

services/device/fingerprint/fingerprint_chromeos_unittest.cc

@@ -6,8 +6,8 @@
 
 #include "base/bind.h"
 #include "base/macros.h"
-#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "chromeos/dbus/biod/fake_biod_client.h"
 #include "mojo/public/cpp/bindings/binding.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ -125,7 +125,7 @@ class FingerprintChromeOSTest : public testing::Test {
   int get_records_results() { return get_records_results_; }
 
  private:
-  base::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
   std::unique_ptr<FingerprintChromeOS> fingerprint_;
   int get_records_results_ = 0;