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

chrome/installer/util/google_chrome_distribution_dummy.cc

 // 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.
 //
 // This file defines dummy implementation of several functions from the
 // BrowserDistribution class for Google Chrome. These functions allow 64-bit
 // Windows Chrome binary to build successfully. Since this binary is only used
 // for Native Client support, most of the install/uninstall functionality is not
 // necessary there.
 
@@ skipping 147 matching lines @@
   return false;
 }
 
 bool GoogleChromeDistribution::ExtractUninstallMetrics(
     const DictionaryValue& root, string16* uninstall_metrics_string) {
   NOTREACHED();
   return false;
 }
 
 bool GoogleChromeDistribution::BuildUninstallMetricsString(
-    DictionaryValue* uninstall_metrics_dict, string16* metrics) {
+    const DictionaryValue* uninstall_metrics_dict, string16* metrics) {
   NOTREACHED();
   return false;
 }