Remove content::NotificationObserver dependency from most Prefs code.

base/prefs/public/pref_observer.h

+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_PREFS_PUBLIC_PREF_OBSERVER_H_
+#define BASE_PREFS_PUBLIC_PREF_OBSERVER_H_
+
+#include <string>
+
+class PrefServiceBase;
+
+// TODO(joi): Switch to base::Callback and remove this.
+class PrefObserver {
+ public:
+  virtual void OnPreferenceChanged(PrefServiceBase* service,
+                                   const std::string& pref_name) = 0;
+};
+
+#endif  // BASE_PREFS_PUBLIC_PREF_OBSERVER_H_