Add missing IWYU message_loop.h includes.

third_party/WebKit/Source/platform/TimerTest.cpp

 // Copyright 2015 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 "platform/Timer.h"
 
 #include <memory>
 #include <queue>
+#include "base/message_loop/message_loop.h"
 #include "platform/scheduler/base/task_queue_impl.h"
 #include "platform/scheduler/child/web_scheduler.h"
 #include "platform/scheduler/child/web_task_runner_impl.h"
 #include "platform/scheduler/renderer/renderer_scheduler_impl.h"
 #include "platform/scheduler/renderer/web_view_scheduler.h"
 #include "platform/testing/TestingPlatformSupport.h"
 #include "platform/wtf/CurrentTime.h"
 #include "platform/wtf/PtrUtil.h"
 #include "platform/wtf/RefCounted.h"
 #include "public/platform/Platform.h"
@@ skipping 702 matching lines @@
                                 &TimerTest::CountingTask);
 
   platform_->RunUntilIdle();
   EXPECT_TRUE(!run_times_.size());
   EXPECT_TRUE(task_runner1->IsEmpty());
   EXPECT_TRUE(task_runner2->IsEmpty());
 }
 
 }  // namespace
 }  // namespace blink