ui: Cleanup class/struct forward declarations

ui/aura/window_tree_host_x11.h

 // Copyright (c) 2012 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_AURA_WINDOW_TREE_HOST_X11_H_
 #define UI_AURA_WINDOW_TREE_HOST_X11_H_
 
 #include <stdint.h>
 
 #include <memory>
 
 #include "base/macros.h"
 #include "ui/aura/aura_export.h"
 #include "ui/aura/window_tree_host.h"
 #include "ui/events/platform/platform_event_dispatcher.h"
 #include "ui/gfx/geometry/insets.h"
 #include "ui/gfx/geometry/rect.h"
 #include "ui/gfx/x/x11_atom_cache.h"
 
 // X forward decls to avoid including Xlib.h in a header file.
 typedef struct _XDisplay XDisplay;
 typedef unsigned long XID;
 typedef XID Window;
 
 namespace ui {
-class MouseEvent;
 class XScopedEventSelector;
 }
 
 namespace aura {
 
 class AURA_EXPORT WindowTreeHostX11 : public WindowTreeHost,
                                       public ui::PlatformEventDispatcher {
  public:
   explicit WindowTreeHostX11(const gfx::Rect& bounds);
   ~WindowTreeHostX11() override;
@@ skipping 69 matching lines @@
 namespace test {
 
 // Set the default value of the override redirect flag used to
 // create a X window for WindowTreeHostX11.
 AURA_EXPORT void SetUseOverrideRedirectWindowByDefault(bool override_redirect);
 
 }  // namespace test
 }  // namespace aura
 
 #endif  // UI_AURA_WINDOW_TREE_HOST_X11_H_