Remove content::NotificationObserver dependency from most Prefs code.

chrome/browser/background/background_mode_manager.h

Index: chrome/browser/background/background_mode_manager.h
diff --git a/chrome/browser/background/background_mode_manager.h b/chrome/browser/background/background_mode_manager.h
index 5a3db3311f8de17bf0c3715b5653eeaa224c16b0..2cb8a7891083694de535b262ccbb361a78a46a89 100644
--- a/chrome/browser/background/background_mode_manager.h
+++ b/chrome/browser/background/background_mode_manager.h
@@ -9,6 +9,7 @@
 
 #include "base/gtest_prod_util.h"
 #include "base/prefs/public/pref_change_registrar.h"
+#include "base/prefs/public/pref_observer.h"
 #include "chrome/browser/background/background_application_list_model.h"
 #include "chrome/browser/profiles/profile_info_cache_observer.h"
 #include "chrome/browser/profiles/profile_keyed_service.h"
@@ -47,6 +48,7 @@ class BackgroundModeManager
     : public content::NotificationObserver,
       public BackgroundApplicationListModel::Observer,
       public ProfileInfoCacheObserver,
+      public PrefObserver,
       public ui::SimpleMenuModel::Delegate {
  public:
   BackgroundModeManager(CommandLine* command_line,
@@ -158,6 +160,10 @@ class BackgroundModeManager
                        const content::NotificationSource& source,
                        const content::NotificationDetails& details) OVERRIDE;
 
+  // PrefObserver implementation.
+  virtual void OnPreferenceChanged(PrefServiceBase* service,
+                                   const std::string& pref_name) OVERRIDE;
+
   // Called when the kBackgroundModeEnabled preference changes.
   void OnBackgroundModeEnabledPrefChanged();