Add missing IWYU message_loop.h includes.

remoting/host/linux/certificate_watcher_unittest.cc

 // Copyright 2016 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 "remoting/host/linux/certificate_watcher.h"
 
 #include <cstdlib>
 #include <memory>
 #include <string>
 
 #include "base/bind.h"
 #include "base/files/file_util.h"
 #include "base/files/scoped_temp_dir.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace remoting {
 
 const char kCertFileName[] = "cert9.db";
 const char kKeyFileName[] = "key4.db";
 const char kPKCSFileName[] = "pkcs11.txt";
 const char kOtherFileName[] = "testfile.txt";
 
@@ skipping 138 matching lines @@
   // NSS DB contents.
   EXPECT_EQ(0, restart_count_);
   Start();
   EXPECT_EQ(0, restart_count_);
   TouchFile(kOtherFileName);
   RunAndWait();
   EXPECT_EQ(0, restart_count_);
 }
 
 } // namespace remoting