Add missing IWYU message_loop.h includes.

components/sync/engine/attachments/fake_attachment_uploader_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/sync/engine/attachments/fake_attachment_uploader.h"
 
 #include "base/bind.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/ref_counted_memory.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "components/sync/model/attachments/attachment.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace syncer {
 
 namespace {
 
 const char kAttachmentData[] = "some data";
 
@@ skipping 31 matching lines @@
   Attachment attachment2 = Attachment::Create(some_data);
   Attachment attachment3 = Attachment::Create(some_data);
   uploader.UploadAttachment(attachment1, upload_callback);
   uploader.UploadAttachment(attachment2, upload_callback);
   uploader.UploadAttachment(attachment3, upload_callback);
   base::RunLoop().RunUntilIdle();
   EXPECT_EQ(upload_callback_count, 3);
 }
 
 }  // namespace syncer