Add missing IWYU message_loop.h includes.

components/autofill/core/browser/autofill_download_manager_unittest.cc

 // Copyright 2014 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 "components/autofill/core/browser/autofill_download_manager.h"
 
 #include <stddef.h>
 
 #include <list>
 #include <memory>
 #include <utility>
 #include <vector>
 
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/test/histogram_tester.h"
 #include "base/test/test_timeouts.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "components/autofill/core/browser/autofill_field.h"
 #include "components/autofill/core/browser/autofill_metrics.h"
 #include "components/autofill/core/browser/autofill_type.h"
@@ skipping 623 matching lines @@
   EXPECT_EQ(0U, responses_.size());
 
   fetcher = factory.GetFetcherByID(3);
   ASSERT_TRUE(fetcher);
   FakeOnURLFetchComplete(fetcher, 200, std::string(responses[0]));
   ASSERT_EQ(1U, responses_.size());
   EXPECT_EQ(responses[0], responses_.front().response);
 }
 
 }  // namespace autofill