ui: Cleanup class/struct forward declarations

ui/ozone/platform/cast/platform_window_cast.h

 // Copyright 2015 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_OZONE_PLATFORM_CAST_PLATFORM_WINDOW_CAST_H_
 #define UI_OZONE_PLATFORM_CAST_PLATFORM_WINDOW_CAST_H_
 
 #include "base/macros.h"
 #include "ui/gfx/geometry/rect.h"
 #include "ui/gfx/native_widget_types.h"
 #include "ui/platform_window/platform_window.h"
 
 namespace ui {
 
+class PlatformWindowDelegate;
+
 class PlatformWindowCast : public PlatformWindow {
  public:
   PlatformWindowCast(PlatformWindowDelegate* delegate, const gfx::Rect& bounds);
   ~PlatformWindowCast() override {}
 
   // PlatformWindow implementation:
   gfx::Rect GetBounds() override;
   void SetBounds(const gfx::Rect& bounds) override;
   void SetTitle(const base::string16& title) override;
   void Show() override {}
@@ skipping 14 matching lines @@
   PlatformWindowDelegate* delegate_;
   gfx::Rect bounds_;
   gfx::AcceleratedWidget widget_;
 
   DISALLOW_COPY_AND_ASSIGN(PlatformWindowCast);
 };
 
 }  // namespace ui
 
 #endif  // UI_OZONE_PLATFORM_CAST_PLATFORM_WINDOW_CAST_H_