Add aura::WindowIdRegistry.

ui/aura/window_id_registry.h

Index: ui/aura/window_id_registry.h
diff --git a/ui/aura/window_id_registry.h b/ui/aura/window_id_registry.h
new file mode 100644
index 0000000000000000000000000000000000000000..595bc865eec1e91aff198a1dbab67b9cfbbea791
--- /dev/null
+++ b/ui/aura/window_id_registry.h
@@ -0,0 +1,48 @@
+// Copyright 2013 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_AURA_WINDOW_ID_REGISTRY_H_
+#define UI_AURA_WINDOW_ID_REGISTRY_H_
+
+#include <map>
+
+#include "base/basictypes.h"
+#include "base/compiler_specific.h"
+#include "base/memory/singleton.h"
+#include "ui/aura/aura_export.h"
+#include "ui/aura/window_observer.h"
+
+namespace aura {
+
+// WindowIdRegistry assigns unique identifiers to Aura windows.
+class AURA_EXPORT WindowIdRegistry : public WindowObserver {
+ public:
+  static WindowIdRegistry* GetInstance();
+
+  // Returns unique Id for |window|. New Id is allocated when this method is
+  // called for the |window| for the first time.
+  int GetIdForWindow(Window* window);
+
+  // Returns window with the specified |id|. NULL is returned if the |id| is
+  // invalid or the window it refers to was destroyed.
+  Window* GetWindowById(int id);
+
+ private:
+  friend struct DefaultSingletonTraits<WindowIdRegistry>;
+  WindowIdRegistry();
+  virtual ~WindowIdRegistry();
+
+  // WindowObserver overrides.
+  virtual void OnWindowDestroying(Window* window) OVERRIDE;
+
+  int next_id_;
+  std::map<Window*, int> window_to_id_map_;
+  std::map<int, Window*> id_to_window_map_;
+
+  DISALLOW_COPY_AND_ASSIGN(WindowIdRegistry);
+};
+
+}  // namespace aura
+
+#endif  // UI_AURA_WINDOW_ID_REGISTRY_H_