Add missing IWYU message_loop.h includes.

components/sync/engine/net/http_bridge_unittest.cc

 // Copyright 2012 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/sync/engine/net/http_bridge.h"
 
 #include <stddef.h>
 
 #include "base/bit_cast.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/strings/stringprintf.h"
 #include "base/threading/thread.h"
 #include "build/build_config.h"
 #include "components/sync/base/cancelation_signal.h"
 #include "net/http/http_response_headers.h"
 #include "net/test/embedded_test_server/embedded_test_server.h"
 #include "net/url_request/test_url_fetcher_factory.h"
 #include "net/url_request/url_request_test_util.h"
@@ skipping 527 matching lines @@
 
   // Sync thread: Finally run the posted task, only to find that our
   // HttpBridgeFactory has been neutered.  Should not crash.
   factory->Init("TestUserAgent", BindToTrackerCallback());
 
   // At this point, attempting to use the factory would trigger a crash.  Both
   // this test and the real world code should make sure this never happens.
 }
 
 }  // namespace syncer