Add missing IWYU message_loop.h includes.

chrome/browser/net/spdyproxy/data_reduction_proxy_chrome_settings_unittest.cc

 // Copyright 2015 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 "chrome/browser/net/spdyproxy/data_reduction_proxy_chrome_settings.h"
 
 #include <memory>
 #include <string>
 
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/test/histogram_tester.h"
 #include "components/data_reduction_proxy/core/browser/data_reduction_proxy_config_test_utils.h"
 #include "components/data_reduction_proxy/core/browser/data_reduction_proxy_test_utils.h"
 #include "components/data_reduction_proxy/core/common/data_reduction_proxy_params.h"
 #include "components/data_reduction_proxy/core/common/data_reduction_proxy_params_test_utils.h"
 #include "components/prefs/pref_registry_simple.h"
 #include "components/prefs/testing_pref_service.h"
 #include "components/proxy_config/proxy_config_pref_names.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ skipping 308 matching lines @@
     EXPECT_EQ("fixed_servers", mode);
     std::string server;
     EXPECT_TRUE(value->GetString("server", &server));
     EXPECT_EQ(test_server, server);
 
     histogram_tester.ExpectUniqueSample(
         "DataReductionProxy.ProxyPrefMigrationResult",
         DataReductionProxyChromeSettings::PROXY_PREF_NOT_CLEARED, 1);
   }
 }