Fix Up SingletonHwnd Observer Lifetime Issues

ui/gfx/win/singleton_hwnd_observer.cc

+// 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.
+
+#include "ui/gfx/win/singleton_hwnd_observer.h"
+
+#include "ui/gfx/win/singleton_hwnd.h"
+
+namespace gfx {
+
+SingletonHwndObserver::SingletonHwndObserver(const WndProc& wnd_proc)
+    : wnd_proc_(wnd_proc) {
+  DCHECK(!wnd_proc.is_null());
+  SingletonHwnd::GetInstance()->AddObserver(this);
+}
+
+SingletonHwndObserver::~SingletonHwndObserver() {
+  ClearWndProc();
+}
+
+void SingletonHwndObserver::ClearWndProc() {
+  if (!wnd_proc_.is_null()) {
+    SingletonHwnd::GetInstance()->RemoveObserver(this);
+    wnd_proc_.Reset();
+  }
+}
+
+void SingletonHwndObserver::OnWndProc(HWND hwnd,
+                                      UINT message,
+                                      WPARAM wparam,
+                                      LPARAM lparam) {
+  wnd_proc_.Run(hwnd, message, wparam, lparam);
+}
+
+}  // namespace gfx