Remove content::NotificationObserver dependency from most Prefs code.

chrome/browser/extensions/extension_service.h

Index: chrome/browser/extensions/extension_service.h
diff --git a/chrome/browser/extensions/extension_service.h b/chrome/browser/extensions/extension_service.h
index 9ad3ab6ba58aca0a0a30386da932aaa5734e6cc7..b67fe8eb60d1224501e85ef0b1fecb2f445c45ec 100644
--- a/chrome/browser/extensions/extension_service.h
+++ b/chrome/browser/extensions/extension_service.h
@@ -17,6 +17,7 @@
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 #include "base/prefs/public/pref_change_registrar.h"
+#include "base/prefs/public/pref_observer.h"
 #include "base/string16.h"
 #include "chrome/browser/extensions/app_shortcut_manager.h"
 #include "chrome/browser/extensions/app_sync_bundle.h"
@@ -140,7 +141,8 @@ class ExtensionServiceInterface : public syncer::SyncableService {
 class ExtensionService
     : public ExtensionServiceInterface,
       public extensions::ExternalProviderInterface::VisitorInterface,
-      public content::NotificationObserver {
+      public content::NotificationObserver,
+      public PrefObserver {
  public:
   // The name of the directory inside the profile where extensions are
   // installed to.
@@ -621,6 +623,10 @@ class ExtensionService
                        const content::NotificationSource& source,
                        const content::NotificationDetails& details) OVERRIDE;
 
+  // PrefObserver
+  virtual void OnPreferenceChanged(PrefServiceBase* service,
+                                   const std::string& pref_name) OVERRIDE;
+
   // Whether there are any apps installed. Component apps are not included.
   bool HasApps() const;