Add a do_not_sync pref to ExtensionPrefs.

chrome/common/extensions/sync_helper.h

 // Copyright 2013 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.
 
 #ifndef CHROME_COMMON_EXTENSIONS_SYNC_HELPER_H_
 #define CHROME_COMMON_EXTENSIONS_SYNC_HELPER_H_
 
 namespace extensions {
 
 class Extension;
 
 namespace sync_helper {
 
+// NOTE: The check in the functions here only considers the data in extension
+// itself, not the environment it is in. To determine whether an extension
+// should be synced, you probably want to use util::ShoudSyncExtension and
+// util::ShouldSyncApp.
+
 // Returns true if the extension should be synced.
 bool IsSyncable(const Extension* extension);
 
 // Returns true if the extension uses the sync bucket of this type.
 bool IsSyncableExtension(const Extension* extension);
 bool IsSyncableApp(const Extension* extension);
 
 }  // namespace sync_helper
 }  // namespace extensions
 
 #endif  // CHROME_COMMON_EXTENSIONS_SYNC_HELPER_H_