Remove windows/mac/ios specific code from //ui

ui/gfx/win/singleton_hwnd.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_GFX_WIN_SINGLETON_HWND_H_
-#define UI_GFX_WIN_SINGLETON_HWND_H_
-
-#include <windows.h>
-#include <vector>
-
-#include "base/basictypes.h"
-#include "base/callback_forward.h"
-#include "base/observer_list.h"
-#include "ui/gfx/win/window_impl.h"
-
-template<typename T> struct DefaultSingletonTraits;
-
-namespace gfx {
-
-// Singleton message-only HWND that allows interested clients to receive WM_*
-// notifications.
-class GFX_EXPORT SingletonHwnd : public WindowImpl {
- public:
-  static SingletonHwnd* GetInstance();
-
-  // Observer interface for receiving Windows WM_* notifications.
-  class Observer {
-   public:
-    virtual void OnWndProc(HWND hwnd,
-                           UINT message,
-                           WPARAM wparam,
-                           LPARAM lparam) = 0;
-  };
-
-  // Add/remove observer to receive WM_* notifications.
-  void AddObserver(Observer* observer);
-  void RemoveObserver(Observer* observer);
-
-  // Windows callback for WM_* notifications.
-  virtual BOOL ProcessWindowMessage(HWND window,
-                                    UINT message,
-                                    WPARAM wparam,
-                                    LPARAM lparam,
-                                    LRESULT& result,
-                                    DWORD msg_map_id) override;
-
- private:
-  friend struct DefaultSingletonTraits<SingletonHwnd>;
-
-  SingletonHwnd();
-  ~SingletonHwnd();
-
-  // List of registered observers.
-  ObserverList<Observer> observer_list_;
-
-  DISALLOW_COPY_AND_ASSIGN(SingletonHwnd);
-};
-
-}  // namespace gfx
-
-#endif  // UI_GFX_WIN_SINGLETON_HWND_H_