Remove content::NotificationObserver dependency from most Prefs code.

chrome/browser/ui/gtk/global_menu_bar.cc

Index: chrome/browser/ui/gtk/global_menu_bar.cc
diff --git a/chrome/browser/ui/gtk/global_menu_bar.cc b/chrome/browser/ui/gtk/global_menu_bar.cc
index 7ea211daf86bd39b80636342278c7eae8925b879..31cf7fcde877149f4dbbf87f2beaa44908211e0d 100644
--- a/chrome/browser/ui/gtk/global_menu_bar.cc
+++ b/chrome/browser/ui/gtk/global_menu_bar.cc
@@ -281,11 +281,8 @@ void GlobalMenuBar::EnabledStateChangedForCommand(int id, bool enabled) {
     gtk_widget_set_sensitive(it->second, enabled);
 }
 
-void GlobalMenuBar::Observe(int type,
-                            const content::NotificationSource& source,
-                            const content::NotificationDetails& details) {
-  DCHECK_EQ(chrome::NOTIFICATION_PREF_CHANGED, type);
-  const std::string& pref_name = *content::Details<std::string>(details).ptr();
+void GlobalMenuBar::OnPreferenceChanged(PrefServiceBase* service,
+                                        const std::string& pref_name) {
   DCHECK_EQ(prefs::kShowBookmarkBar, pref_name);
   OnBookmarkBarVisibilityChanged();
 }