Add missing IWYU message_loop.h includes.

chromeos/dbus/biod/biod_client_unittest.cc

 // Copyright 2017 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 "chromeos/dbus/biod/biod_client.h"
 
 #include <map>
 #include <string>
 
 #include "base/bind.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/strings/stringprintf.h"
 #include "chromeos/dbus/biod/messages.pb.h"
 #include "chromeos/dbus/biod/test_utils.h"
 #include "dbus/mock_bus.h"
 #include "dbus/mock_object_proxy.h"
 #include "dbus/object_path.h"
 #include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ skipping 401 matching lines @@
   client_->RemoveObserver(&observer);
 
   EmitEnrollScanDoneSignal(scan_signal, enroll_session_complete,
                            percent_complete);
   EmitAuthScanDoneSignal(scan_signal, test_attempt);
   EXPECT_EQ(1, observer.NumEnrollScansReceived());
   EXPECT_EQ(1, observer.NumAuthScansReceived());
   EXPECT_EQ(1, observer.num_failures_received());
 }
 }  // namespace chromeos