ui: Cleanup class/struct forward declarations

ui/views/mus/mus_client.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 UI_VIEWS_MUS_MUS_CLIENT_INIT_H_
 #define UI_VIEWS_MUS_MUS_CLIENT_INIT_H_
 
 #include <string>
 
 #include "base/macros.h"
 #include "ui/aura/mus/window_tree_client_delegate.h"
 #include "ui/base/dragdrop/os_exchange_data_provider_factory.h"
 #include "ui/views/mus/mus_export.h"
 #include "ui/views/mus/screen_mus_delegate.h"
 #include "ui/views/widget/widget.h"
 
 namespace aura {
-class Env;
 class GpuService;
 class PropertyConverter;
 class MusContextFactory;
 class Window;
 class WindowTreeClient;
 }
 
 namespace base {
 class SingleThreadTaskRunner;
 }
@@ skipping 94 matching lines @@
   std::unique_ptr<aura::GpuService> gpu_service_;
 
   std::unique_ptr<aura::MusContextFactory> compositor_context_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(MusClient);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_MUS_MUS_CLIENT_INIT_H_