Add missing IWYU message_loop.h includes.

third_party/WebKit/public/platform/scheduler/test/mock_renderer_scheduler.h

 // 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.
 
 #ifndef THIRD_PARTY_WEBKIT_PUBLIC_PLATFORM_SCHEDULER_TEST_MOCK_RENDERER_SCHEDULER_H_
 #define THIRD_PARTY_WEBKIT_PUBLIC_PLATFORM_SCHEDULER_TEST_MOCK_RENDERER_SCHEDULER_H_
 
 #include "base/macros.h"
+#include "base/message_loop/message_loop.h"
 #include "cc/output/begin_frame_args.h"
 #include "public/platform/WebThread.h"
 #include "public/platform/scheduler/renderer/renderer_scheduler.h"
 #include "testing/gmock/include/gmock/gmock.h"
 
 namespace blink {
 namespace scheduler {
 
 class MockRendererScheduler : public RendererScheduler {
  public:
@@ skipping 42 matching lines @@
   MOCK_METHOD1(MainThreadSeemsUnresponsive, bool(base::TimeDelta));
 
  private:
   DISALLOW_COPY_AND_ASSIGN(MockRendererScheduler);
 };
 
 }  // namespace scheduler
 }  // namespace blink
 
 #endif  // THIRD_PARTY_WEBKIT_PUBLIC_PLATFORM_SCHEDULER_TEST_MOCK_RENDERER_SCHEDULER_H_