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);
+
+  if (message == WM_NCDESTROY)
+    ClearWndProc();

[sky, 2015/04/28 23:35:04]

Having any code after Run is dangerous as the callback may end up deleting
'this'. In fact a number of the callbacks do this now.

[robliao, 2015/04/29 00:25:43]

The original implementation was resilient against this. I'm okay with observers
not forwarding WM_NCDESTROY.

+}
+
+}  // namespace gfx