Add missing IWYU message_loop.h includes.

components/data_reduction_proxy/core/browser/data_reduction_proxy_metrics_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/data_reduction_proxy/core/browser/data_reduction_proxy_metrics.h"
 
 #include <memory>
 #include <vector>
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
+#include "base/message_loop/message_loop.h"
 #include "base/time/time.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_test_utils.h"
 #include "components/data_reduction_proxy/core/common/data_reduction_proxy_server.h"
 #include "net/base/load_flags.h"
 #include "net/log/net_log_source_type.h"
 #include "net/log/net_log_with_source.h"
 #include "net/proxy/proxy_server.h"
 #include "net/proxy/proxy_service.h"
@@ skipping 173 matching lines @@
     }
 
     EXPECT_EQ(test_case.expected_request_type,
               GetDataReductionProxyRequestType(
                   *request, data_reduction_proxy_config,
                   *test_context->config()));
   }
 }
 
 }  // namespace data_reduction_proxy