Remove content::NotificationObserver dependency from most Prefs code.

chrome/browser/chromeos/audio/audio_handler.h

Index: chrome/browser/chromeos/audio/audio_handler.h
diff --git a/chrome/browser/chromeos/audio/audio_handler.h b/chrome/browser/chromeos/audio/audio_handler.h
index ecb9f57af27d96219653abff70897bbac2c76295..1b77f9fc1d3469582a409b03c0490458b6c2bdaf 100644
--- a/chrome/browser/chromeos/audio/audio_handler.h
+++ b/chrome/browser/chromeos/audio/audio_handler.h
@@ -9,11 +9,8 @@
 #include "base/memory/scoped_ptr.h"
 #include "base/observer_list.h"
 #include "base/prefs/public/pref_change_registrar.h"
+#include "base/prefs/public/pref_observer.h"
 #include "base/threading/thread.h"
-#include "content/public/browser/notification_details.h"
-#include "content/public/browser/notification_observer.h"
-#include "content/public/browser/notification_registrar.h"
-#include "content/public/browser/notification_source.h"
 
 template <typename T> struct DefaultSingletonTraits;
 
@@ -24,7 +21,7 @@ namespace chromeos {
 
 class AudioMixer;
 
-class AudioHandler : public content::NotificationObserver {
+class AudioHandler : public PrefObserver {
  public:
   class VolumeObserver {
    public:
@@ -76,10 +73,9 @@ class AudioHandler : public content::NotificationObserver {
   void AddVolumeObserver(VolumeObserver* observer);
   void RemoveVolumeObserver(VolumeObserver* observer);
 
-  // Overridden from content::NotificationObserver:
-  virtual void Observe(int type,
-                       const content::NotificationSource& source,
-                       const content::NotificationDetails& details) OVERRIDE;
+  // Overridden from PrefObserver:
+  virtual void OnPreferenceChanged(PrefServiceBase* service,
+                                   const std::string& pref_name) OVERRIDE;
 
  private:
   // Defines the delete on exit Singleton traits we like.  Best to have this
@@ -107,7 +103,6 @@ class AudioHandler : public content::NotificationObserver {
   PrefService* local_state_;  // not owned
 
   PrefChangeRegistrar pref_change_registrar_;
-  content::NotificationRegistrar registrar_;
 
   DISALLOW_COPY_AND_ASSIGN(AudioHandler);
 };