Fix Up SingletonHwnd Observer Lifetime Issues

chrome/browser/extensions/global_shortcut_listener_win.cc

Index: chrome/browser/extensions/global_shortcut_listener_win.cc
diff --git a/chrome/browser/extensions/global_shortcut_listener_win.cc b/chrome/browser/extensions/global_shortcut_listener_win.cc
index 8f6f1912e36fc0b7a63893473c190c32798d1b9c..ad12f859d34ddd99ad3cd92cecfd5d101c58ccda 100644
--- a/chrome/browser/extensions/global_shortcut_listener_win.cc
+++ b/chrome/browser/extensions/global_shortcut_listener_win.cc
@@ -4,11 +4,14 @@
 
 #include "chrome/browser/extensions/global_shortcut_listener_win.h"
 
+#include "base/bind.h"
+#include "base/bind_helpers.h"
 #include "base/win/win_util.h"
 #include "content/public/browser/browser_thread.h"
 #include "ui/base/accelerators/accelerator.h"
 #include "ui/events/event_constants.h"
 #include "ui/events/keycodes/keyboard_code_conversion_win.h"
+#include "ui/gfx/win/singleton_hwnd.h"
 
 using content::BrowserThread;
 
@@ -35,21 +38,23 @@ GlobalShortcutListenerWin::~GlobalShortcutListenerWin() {
 void GlobalShortcutListenerWin::StartListening() {
   DCHECK(!is_listening_);  // Don't start twice.
   DCHECK(!hotkey_ids_.empty());  // Also don't start if no hotkey is registered.
-  gfx::SingletonHwnd::GetInstance()->AddObserver(this);
+  singleton_hwnd_observer_.reset(new gfx::SingletonHwndObserver(
+      base::Bind(
+          &GlobalShortcutListenerWin::OnHotkey, base::Unretained(this))));

[sky, 2015/04/28 13:20:34]

You should document why unretained is safe here.

[robliao, 2015/04/28 14:11:55]

This use of base::Unretained(this) is fairly idiomatic with an understanding
that the lifetime is the same as the holder. This is used in 1000+ occurrences
of the codebase without a safety comment.

We shouldn't get into the business of having all of these classes support
refcounting just to support this callback.

[robliao, 2015/04/28 14:12:38]

lifetime same or longer than the holder.

   is_listening_ = true;
 }
 
 void GlobalShortcutListenerWin::StopListening() {
   DCHECK(is_listening_);  // No point if we are not already listening.
   DCHECK(hotkey_ids_.empty());  // Make sure the map is clean before ending.
-  gfx::SingletonHwnd::GetInstance()->RemoveObserver(this);
+  singleton_hwnd_observer_.reset(nullptr);
   is_listening_ = false;
 }
 
-void GlobalShortcutListenerWin::OnWndProc(HWND hwnd,
-                                          UINT message,
-                                          WPARAM wparam,
-                                          LPARAM lparam) {
+void GlobalShortcutListenerWin::OnHotkey(HWND hwnd,
+                                         UINT message,
+                                         WPARAM wparam,
+                                         LPARAM lparam) {
   if (message != WM_HOTKEY)
     return;