Fix Up SingletonHwnd Observer Lifetime Issues

ui/gfx/win/singleton_hwnd_observer.h

+// Copyright 2015 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_OBSERVER_H_
+#define UI_GFX_WIN_SINGLETON_HWND_OBSERVER_H_
+
+#include <windows.h>
+
+#include "base/callback.h"
+#include "base/macros.h"
+#include "ui/gfx/gfx_export.h"
+
+namespace gfx {
+
+class SingletonHwnd;
+
+// Singleton lifetime management is tricky. This observer handles the correct
+// cleanup if either the SingletonHwnd or forwarded object is destroyed first.
+class GFX_EXPORT SingletonHwndObserver {
+ public:
+  typedef base::Callback<void(HWND, UINT, WPARAM, LPARAM)> WndProc;
+
+  explicit SingletonHwndObserver(const WndProc& wnd_proc);
+  ~SingletonHwndObserver();
+
+ private:
+  friend class SingletonHwnd;
+
+  void ClearWndProc();
+  void OnWndProc(HWND hwnd, UINT message, WPARAM wparam, LPARAM lparam);
+
+  WndProc wnd_proc_;
+
+  DISALLOW_COPY_AND_ASSIGN(SingletonHwndObserver);
+};
+
+}  // namespace gfx
+
+#endif  // UI_GFX_WIN_SINGLETON_HWND_OBSERVER_H_