Do not allow values to be migrated without MACs if the destination already has a MAC.

chrome/browser/prefs/tracked/tracked_preferences_migration.cc

 // Copyright 2014 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/tracked/tracked_preferences_migration.h"
 
 #include "base/bind.h"
 #include "base/callback.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
@@ skipping 174 matching lines @@
         old_hash_store_contents->Get(pref_name, &old_hash);
       if (!old_hash && legacy_hash_store_contents) {
         legacy_hash_store_contents->Get(pref_name, &old_hash);
         if (old_hash)
           *used_legacy_pref_hashes = true;
       }
       if (old_hash) {
         new_hash_store_transaction->ImportHash(pref_name, old_hash);
         *new_store_altered = true;
       } else if (!destination_hash_missing) {
-        new_hash_store_transaction->ClearHash(pref_name);
+        // Do not allow values to be migrated without MACs if the destination
+        // already has a MAC (http://crbug.com/414554). Remove the migrated
+        // value in order to provide the same no-op behaviour as if the pref was
+        // added to the wrong file when there was already a value for
+        // |pref_name| in |new_store|.
+        new_store->Remove(pref_name, NULL);
         *new_store_altered = true;
       }
     }
   }
 }
 
 TrackedPreferencesMigrator::TrackedPreferencesMigrator(
     const std::set<std::string>& unprotected_pref_names,
     const std::set<std::string>& protected_pref_names,
     const base::Callback<void(const std::string& key)>&
@@ skipping 150 matching lines @@
           unprotected_store_cleaner,
           protected_store_cleaner,
           register_on_successful_unprotected_store_write_callback,
           register_on_successful_protected_store_write_callback,
           unprotected_pref_hash_store.Pass(),
           protected_pref_hash_store.Pass(),
           legacy_pref_hash_store.Pass(),
           unprotected_pref_filter,
           protected_pref_filter));
 }