CrOS settings/metrics: Correctly store Subscriptions

chrome/browser/chromeos/settings/cros_settings.h

 // Copyright (c) 2012 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 CHROME_BROWSER_CHROMEOS_SETTINGS_CROS_SETTINGS_H_
 #define CHROME_BROWSER_CHROMEOS_SETTINGS_CROS_SETTINGS_H_
 
 #include <memory>
 #include <string>
 #include <vector>
@@ skipping 94 matching lines @@
   bool FindEmailInList(const std::string& path,
                        const std::string& email,
                        bool* wildcard_match) const;
 
   // Adding/removing of providers.
   bool AddSettingsProvider(std::unique_ptr<CrosSettingsProvider> provider);
   std::unique_ptr<CrosSettingsProvider> RemoveSettingsProvider(
       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;
 
  private:
   friend class CrosSettingsTest;
 
   // Fires system setting change callback.
   void FireObservers(const std::string& path);
 
@@ skipping 15 matching lines @@
   ScopedTestCrosSettings();
   ~ScopedTestCrosSettings();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ScopedTestCrosSettings);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_SETTINGS_CROS_SETTINGS_H_