Remove content::NotificationObserver dependency from most Prefs code.

base/base.gyp

 # 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.
 
 {
   'variables': {
     'chromium_code': 1,
   },
   'includes': [
     '../build/win_precompile.gypi',
@@ skipping 305 matching lines @@
       ],
       'sources': [
         'prefs/default_pref_store.cc',
         'prefs/default_pref_store.h',
         'prefs/json_pref_store.cc',
         'prefs/json_pref_store.h',
         'prefs/overlay_user_pref_store.cc',
         'prefs/overlay_user_pref_store.h',
         'prefs/persistent_pref_store.h',
         'prefs/pref_notifier.h',
+        'prefs/pref_observer.h',
         'prefs/pref_store.cc',
         'prefs/pref_store.h',
         'prefs/pref_value_map.cc',
         'prefs/pref_value_map.h',
         'prefs/public/pref_change_registrar.cc',
         'prefs/public/pref_change_registrar.h',
         'prefs/public/pref_service_base.h',
         'prefs/value_map_pref_store.cc',
         'prefs/value_map_pref_store.h',
       ],
@@ skipping 757 matching lines @@
             'base_unittests.isolate',
           ],
           'sources': [
             'base_unittests.isolate',
           ],
         },
       ],
     }],
   ],
 }