Add missing IWYU message_loop.h includes.

cc/test/layer_tree_test.cc

 // Copyright 2011 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 "cc/test/layer_tree_test.h"
 
 #include "base/command_line.h"
 #include "base/location.h"
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "cc/animation/animation.h"
 #include "cc/animation/animation_host.h"
 #include "cc/animation/animation_player.h"
 #include "cc/animation/timing_function.h"
 #include "cc/base/switches.h"
 #include "cc/input/input_handler.h"
 #include "cc/layers/layer.h"
@@ skipping 879 matching lines @@
   DCHECK(task_runner_provider()->IsMainThread() ||
          task_runner_provider()->IsMainThreadBlocked());
   return layer_tree_host_.get();
 }
 
 Proxy* LayerTreeTest::proxy() {
   return layer_tree_host() ? layer_tree_host()->proxy() : NULL;
 }
 
 }  // namespace cc