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

ui/gfx/win/singleton_hwnd.cc

-// 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.
-
-#include "ui/gfx/win/singleton_hwnd.h"
-
-#include "base/memory/singleton.h"
-#include "base/message_loop/message_loop.h"
-
-namespace gfx {
-
-// static
-SingletonHwnd* SingletonHwnd::GetInstance() {
-  return Singleton<SingletonHwnd>::get();
-}
-
-void SingletonHwnd::AddObserver(Observer* observer) {
-  observer_list_.AddObserver(observer);
-}
-
-void SingletonHwnd::RemoveObserver(Observer* observer) {
-  if (!hwnd())
-    return;
-  observer_list_.RemoveObserver(observer);
-}
-
-BOOL SingletonHwnd::ProcessWindowMessage(HWND window,
-                                         UINT message,
-                                         WPARAM wparam,
-                                         LPARAM lparam,
-                                         LRESULT& result,
-                                         DWORD msg_map_id) {
-  FOR_EACH_OBSERVER(Observer,
-                    observer_list_,
-                    OnWndProc(window, message, wparam, lparam));
-  return false;
-}
-
-SingletonHwnd::SingletonHwnd() {
-  if (!base::MessageLoopForUI::IsCurrent()) {
-    // Creating this window in (e.g.) a renderer inhibits shutdown on
-    // Windows. See http://crbug.com/230122 and http://crbug.com/236039.
-    DLOG(ERROR) << "Cannot create windows on non-UI thread!";
-    return;
-  }
-  WindowImpl::Init(NULL, Rect());
-}
-
-SingletonHwnd::~SingletonHwnd() {
-}
-
-}  // namespace gfx