Correct const accessors in base/values.(h|cc)

printing/page_size_margins.cc

-// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// 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.
 
 #include "printing/page_size_margins.h"
 
 #include "base/logging.h"
 #include "base/values.h"
 #include "printing/print_job_constants.h"
 
 namespace printing {
 
 void GetCustomMarginsFromJobSettings(const base::DictionaryValue& settings,
                                      PageSizeMargins* page_size_margins) {
-  DictionaryValue* custom_margins;
+  const DictionaryValue* custom_margins;
   if (!settings.GetDictionary(kSettingMarginsCustom, &custom_margins) ||
       !custom_margins->GetDouble(kSettingMarginTop,
                                  &page_size_margins->margin_top) ||
       !custom_margins->GetDouble(kSettingMarginBottom,
                                  &page_size_margins->margin_bottom) ||
       !custom_margins->GetDouble(kSettingMarginLeft,
                                  &page_size_margins->margin_left) ||
       !custom_margins->GetDouble(kSettingMarginRight,
                                  &page_size_margins->margin_right)) {
     NOTREACHED();
   }
 }
 
 }  // namespace printing