Use ScopedTaskEnvironment instead of MessageLoop in /chromeos/components/tether

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=hansberry@chromium.org

Change-Id: I3659594870f528f8fa4523aeace161c246614ab2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1649372
Auto-Submit: Carlos Caballero <carlscab@google.com>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Commit-Queue: Ryan Hansberry <hansberry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667306}

chromeos/components/tether/active_host_network_state_updater_unittest.cc

@@ -8,7 +8,7 @@
 
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
-#include "base/message_loop/message_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "chromeos/components/tether/fake_active_host.h"
 #include "chromeos/network/network_state.h"
 #include "chromeos/network/network_state_handler.h"
@@ -98,7 +98,7 @@ class ActiveHostNetworkStateUpdaterTest : public testing::Test {
     EXPECT_TRUE(network_state->IsConnectedState());
   }
 
-  base::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
 
   NetworkStateTestHelper helper_{true /* use_default_devices_and_services */};
 

chromeos/components/tether/tether_connector_impl_unittest.cc

@@ -8,8 +8,8 @@
 
 #include "base/bind.h"
 #include "base/memory/ptr_util.h"
-#include "base/message_loop/message_loop.h"
 #include "base/test/metrics/histogram_tester.h"
+#include "base/test/scoped_task_environment.h"
 #include "chromeos/components/multidevice/remote_device_ref.h"
 #include "chromeos/components/multidevice/remote_device_test_util.h"
 #include "chromeos/components/tether/connect_tethering_operation.h"
@@ -313,7 +313,7 @@ class TetherConnectorImplTest : public testing::Test {
   }
 
   const multidevice::RemoteDeviceRefList test_devices_;
-  const base::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
   NetworkStateTestHelper helper_{true /* use_default_devices_and_services */};
 
   std::unique_ptr<FakeConnectTetheringOperationFactory> fake_operation_factory_;