platform-window: Add a PlatformWindowFactory for creating windows.

ui/platform_window/platform_window_factory.cc

+// 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.
+
+#include "ui/platform_window/platform_window_factory.h"
+
+#include "base/at_exit.h"
+#include "base/bind.h"
+#include "base/logging.h"
+
+namespace ui {
+
+namespace {
+
+PlatformWindowFactory* instance = NULL;
+
+}  // namespace
+
+PlatformWindowFactory::PlatformWindowFactory() {
+  CHECK(!instance) << "There can only be a single PlatformWindowFactory.";
+  instance = this;
+  base::AtExitManager::RegisterTask(
+      base::Bind(&base::DeletePointer<PlatformWindowFactory>, instance));
+}
+
+PlatformWindowFactory::~PlatformWindowFactory() {
+  CHECK_EQ(instance, this);
+  instance = NULL;
+}
+
+// static
+PlatformWindowFactory* PlatformWindowFactory::GetInstance() {
+  return instance;
+}
+
+}  // namespace ui