Remove //chrome dependency from ExtensionPrefs

extensions/extensions.gyp

 # Copyright 2013 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,
   },
   'targets': [
     {
@@ skipping 269 matching lines @@
         'browser/extension_pref_store.cc',
         'browser/extension_pref_store.h',
         'browser/extension_pref_value_map.cc',
         'browser/extension_pref_value_map_factory.cc',
         'browser/extension_pref_value_map_factory.h',
         'browser/extension_pref_value_map.h',
         'browser/extension_prefs.cc',
         'browser/extension_prefs.h',
         'browser/extension_prefs_factory.cc',
         'browser/extension_prefs_factory.h',
+        'browser/extension_prefs_observer.h',
         'browser/extension_prefs_scope.h',
         'browser/extension_registry.cc',
         'browser/extension_registry.h',
         'browser/extension_registry_factory.cc',
         'browser/extension_registry_factory.h',
         'browser/extension_registry_observer.h',
         'browser/extension_scoped_prefs.h',
         'browser/extension_system.cc',
         'browser/extension_system.h',
         'browser/extension_system_provider.cc',
@@ skipping 85 matching lines @@
         'common/test_util.cc',
         'common/test_util.h',
         'common/value_builder.cc',
         'common/value_builder.h',
       ],
       # Disable c4267 warnings until we fix size_t to int truncations.
       'msvs_disabled_warnings': [ 4267, ],
     },
   ]
 }