CrOS settings/metrics: Correctly store Subscriptions

chrome/browser/chromeos/settings/cros_settings.h

Index: chrome/browser/chromeos/settings/cros_settings.h
diff --git a/chrome/browser/chromeos/settings/cros_settings.h b/chrome/browser/chromeos/settings/cros_settings.h
index 8b39e7d9a520817be15bb68dff57ba2fe07ff5c9..adeb416adac238ccea0fa510ce1fb0851b44aeae 100644
--- a/chrome/browser/chromeos/settings/cros_settings.h
+++ b/chrome/browser/chromeos/settings/cros_settings.h
@@ -112,10 +112,10 @@ class CrosSettings : public base::NonThreadSafe {
       CrosSettingsProvider* provider);
 
   // Add an observer Callback for changes for the given |path|.
-  typedef base::CallbackList<void(void)>::Subscription ObserverSubscription;
+  using ObserverSubscription = base::CallbackList<void(void)>::Subscription;
   std::unique_ptr<ObserverSubscription> AddSettingsObserver(
       const std::string& path,
-      const base::Closure& callback);
+      const base::Closure& callback) WARN_UNUSED_RESULT;
 
   // Returns the provider that handles settings with the |path| or prefix.
   CrosSettingsProvider* GetProvider(const std::string& path) const;