Keep emtpy List/Dictionary pref value with non-empty default.

chrome/browser/prefs/overlay_user_pref_store.h

 // 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.
 
 #ifndef CHROME_BROWSER_PREFS_OVERLAY_USER_PREF_STORE_H_
 #define CHROME_BROWSER_PREFS_OVERLAY_USER_PREF_STORE_H_
 #pragma once
 
 #include <map>
 #include <string>
@@ skipping 27 matching lines @@
   virtual ReadResult GetValue(const std::string& key,
                               const base::Value** result) const OVERRIDE;
 
   // Methods of PersistentPrefStore.
   virtual ReadResult GetMutableValue(const std::string& key,
                                      base::Value** result) OVERRIDE;
   virtual void SetValue(const std::string& key, base::Value* value) OVERRIDE;
   virtual void SetValueSilently(const std::string& key,
                                 base::Value* value) OVERRIDE;
   virtual void RemoveValue(const std::string& key) OVERRIDE;
+  virtual void MarkNeedsEmptyValue(const std::string& key) OVERRIDE;
   virtual bool ReadOnly() const OVERRIDE;
   virtual PrefReadError GetReadError() const OVERRIDE;
   virtual PrefReadError ReadPrefs() OVERRIDE;
   virtual void ReadPrefsAsync(ReadErrorDelegate* delegate) OVERRIDE;
   virtual void CommitPendingWrite() OVERRIDE;
   virtual void ReportValueChanged(const std::string& key) OVERRIDE;
 
   void RegisterOverlayPref(const std::string& key);
   void RegisterOverlayPref(const std::string& overlay_key,
                            const std::string& underlay_key);
@@ skipping 15 matching lines @@
   ObserverList<PrefStore::Observer, true> observers_;
   PrefValueMap overlay_;
   scoped_refptr<PersistentPrefStore> underlay_;
   NamesMap overlay_to_underlay_names_map_;
   NamesMap underlay_to_overlay_names_map_;
 
   DISALLOW_COPY_AND_ASSIGN(OverlayUserPrefStore);
 };
 
 #endif  // CHROME_BROWSER_PREFS_OVERLAY_USER_PREF_STORE_H_