ui: Cleanup class/struct forward declarations

extensions/browser/app_window/native_app_window.h

 // Copyright 2014 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 EXTENSIONS_BROWSER_APP_WINDOW_NATIVE_APP_WINDOW_H_
 #define EXTENSIONS_BROWSER_APP_WINDOW_NATIVE_APP_WINDOW_H_
 
 #include <memory>
 #include <vector>
 
 #include "components/web_modal/web_contents_modal_dialog_host.h"
 #include "third_party/skia/include/core/SkColor.h"
 #include "ui/base/base_window.h"
 #include "ui/gfx/geometry/insets.h"
 
+class SkRegion;
+
 namespace content {
 struct NativeWebKeyboardEvent;
 }
 
 namespace extensions {
 
 struct DraggableRegion;
 
 // This is an interface to a native implementation of a app window, used for
 // new-style packaged apps. App windows contain a web contents, but no tabs
@@ skipping 65 matching lines @@
   // Returns false if the underlying native window ignores alpha transparency
   // when compositing.
   virtual bool CanHaveAlphaEnabled() const = 0;
 
   ~NativeAppWindow() override {}
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_APP_WINDOW_NATIVE_APP_WINDOW_H_