Refactor FileBasedPolicyProvider, introduce AsynchronousPolicyProvider.

chrome/browser/policy/configuration_policy_provider.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/policy/configuration_policy_provider.h"
 
 #include "base/values.h"
 #include "chrome/common/notification_service.h"
 
 namespace policy {
 
 ConfigurationPolicyProvider::ConfigurationPolicyProvider(
     const PolicyDefinitionList* policy_list)
     : policy_definition_list_(policy_list) {
 }
 
 ConfigurationPolicyProvider::~ConfigurationPolicyProvider() {}
 
 void ConfigurationPolicyProvider::NotifyStoreOfPolicyChange() {
   NotificationService::current()->Notify(
       NotificationType::POLICY_CHANGED,
       Source<ConfigurationPolicyProvider>(this),
       NotificationService::NoDetails());
 }
 
 void ConfigurationPolicyProvider::DecodePolicyValueTree(
-    DictionaryValue* policies,
+    const DictionaryValue* policies,
     ConfigurationPolicyStoreInterface* store) {
   const PolicyDefinitionList* policy_list(policy_definition_list());
   for (const PolicyDefinitionList::Entry* i = policy_list->begin;
        i != policy_list->end; ++i) {
     Value* value;
     if (policies->Get(i->name, &value) && value->IsType(i->value_type))
       store->Apply(i->policy_type, value->DeepCopy());
   }
 
   // TODO(mnissler): Handle preference overrides once |ConfigurationPolicyStore|
   // supports it.
 }
 
 }  // namespace policy