Fix broken PrefService::preference_set() returning not all registered preferences.

chrome/browser/prefs/value_map_pref_store.cc

 // Copyright (c) 2010 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.
 
 #include "chrome/browser/prefs/value_map_pref_store.h"
 
 #include <algorithm>
 
 #include "base/stl_util-inl.h"
 #include "base/values.h"
@@ skipping 21 matching lines @@
 }
 
 void ValueMapPrefStore::RemoveValue(const std::string& key) {
   if (prefs_.RemoveValue(key))
     FOR_EACH_OBSERVER(Observer, observers_, OnPrefValueChanged(key));
 }
 
 void ValueMapPrefStore::NotifyInitializationCompleted() {
   FOR_EACH_OBSERVER(Observer, observers_, OnInitializationCompleted());
 }
+
+ValueMapPrefStore::iterator ValueMapPrefStore::begin() {
+  return prefs_.begin();
+}
+
+ValueMapPrefStore::iterator ValueMapPrefStore::end() {
+  return prefs_.end();
+}
+
+ValueMapPrefStore::const_iterator ValueMapPrefStore::begin() const {
+  return prefs_.begin();
+}
+
+ValueMapPrefStore::const_iterator ValueMapPrefStore::end() const {
+  return prefs_.end();
+}