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,

[jam, 2012/10/30 16:17:41]

nit: so do do we really need the pref service pointer here?

[Mattias Nissler (ping if slow), 2012/10/30 16:32:33]

I don't think so, but Jói has looked at all the implementations recently ;) In
the final base::Callback-based solution, I'd prefer to use no arguments (i.e. a
base::Closure), as that will give us the most flexibility. For callers that need
the name, they can Bind it when constructing the callback.

[Jói, 2012/10/30 16:37:59]

Agreed, in the final implementation it will be a base::Callback<void(void)>,
although some callers will definitely want to bind parameters at registration
time.

+                                   const std::string& pref_name) = 0;
+};
+
+#endif  // BASE_PREFS_PUBLIC_PREF_OBSERVER_H_