Add missing IWYU message_loop.h includes.

components/update_client/ping_manager_unittest.cc

 // Copyright 2013 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/update_client/ping_manager.h"
 
 #include <memory>
 #include <string>
 #include <vector>
 
 #include "base/memory/ptr_util.h"
 #include "base/memory/ref_counted.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/version.h"
 #include "components/update_client/component.h"
 #include "components/update_client/test_configurator.h"
 #include "components/update_client/update_engine.h"
 #include "components/update_client/url_request_post_interceptor.h"
 #include "net/url_request/url_request_test_util.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ skipping 191 matching lines @@
   }
 
   {
     // Tests that the default for |requires_network_encryption| is true.
     Component component(*update_context, "abc");
     EXPECT_FALSE(ping_manager_->SendPing(component));
   }
 }
 
 }  // namespace update_client