Remove the last usages of PrefObserver outside of Prefs.

base/prefs/public/pref_change_registrar.cc

Index: base/prefs/public/pref_change_registrar.cc
diff --git a/base/prefs/public/pref_change_registrar.cc b/base/prefs/public/pref_change_registrar.cc
index 41b422d2ce5623c4e7a6c5f12fb0543951edc4be..681d91200d8f1b303c25332970a0ae579d3cd8ab 100644
--- a/base/prefs/public/pref_change_registrar.cc
+++ b/base/prefs/public/pref_change_registrar.cc
@@ -23,12 +23,6 @@ void PrefChangeRegistrar::Init(PrefServiceBase* service) {
   service_ = service;
 }
 
-void PrefChangeRegistrar::Add(const char* path, PrefObserver* obs) {
-  DCHECK(obs);
-  return Add(path, base::Bind(&PrefObserver::OnPreferenceChanged,
-                              base::Unretained(obs), service_));
-}
-
 void PrefChangeRegistrar::Add(const char* path,
                               const base::Closure& obs) {
   Add(path, base::Bind(&PrefChangeRegistrar::InvokeUnnamedCallback, obs));