compositor_unittests target is unimplmented on Mac

ui/gfx/compositor/test_compositor_host.h

 // Copyright (c) 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.
 
 #ifndef UI_GFX_COMPOSITOR_TEST_COMPOSITOR_HOST_H_
 #define UI_GFX_COMPOSITOR_TEST_COMPOSITOR_HOST_H_
 #pragma once
 
 #include "base/message_loop.h"
 
 namespace gfx {
 class Rect;
 }
 
 namespace ui {
 
 class Compositor;
 
+#if defined (OS_MACOSX)
+class TestCompositorHost {
+#else
 class TestCompositorHost : public MessageLoop::Dispatcher {
+#endif
  public:
   virtual ~TestCompositorHost() {}
 
   // Creates a new TestCompositorHost. The caller owns the returned value.
   static TestCompositorHost* Create(const gfx::Rect& bounds);
 
   // Shows the TestCompositorHost.
   virtual void Show() = 0;
 
   virtual ui::Compositor* GetCompositor() = 0;
 };
 
 }  // namespace ui
 
 #endif  // UI_GFX_COMPOSITOR_TEST_COMPOSITOR_HOST_H_