Add missing IWYU message_loop.h includes.

device/geolocation/location_arbitrator_unittest.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "device/geolocation/location_arbitrator.h"
 
 #include <memory>
 #include <utility>
 
 #include "base/bind.h"
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "device/geolocation/fake_access_token_store.h"
 #include "device/geolocation/fake_location_provider.h"
 #include "device/geolocation/geolocation_delegate.h"
 #include "device/geolocation/geoposition.h"
 #include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 using ::testing::NiceMock;
 
 namespace device {
@@ skipping 417 matching lines @@
 
   // Advance the time a short while to simulate successive calls.
   AdvanceTimeNow(base::TimeDelta::FromMilliseconds(5));
 
   // Update with a less accurate position to verify 240956.
   SetPositionFix(cell(), 3, 139, 150);
   CheckLastPositionInfo(3, 139, 150);
 }
 
 }  // namespace device