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

chrome/browser/prefs/testing_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_TESTING_PREF_STORE_H_
 #define CHROME_BROWSER_PREFS_TESTING_PREF_STORE_H_
 #pragma once
 
 #include <string>
 
@@ skipping 20 matching lines @@
   virtual bool IsInitializationComplete() const OVERRIDE;
 
   // PersistentPrefStore overrides:
   virtual ReadResult GetMutableValue(const std::string& key,
                                      base::Value** result) OVERRIDE;
   virtual void ReportValueChanged(const std::string& key) 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 PersistentPrefStore::PrefReadError ReadPrefs() OVERRIDE;
   virtual void ReadPrefsAsync(ReadErrorDelegate* error_delegate) OVERRIDE;
   virtual void CommitPendingWrite() OVERRIDE {}
 
   // Marks the store as having completed initialization.
   void SetInitializationCompleted();
 
   // Used for tests to trigger notifications explicitly.
@@ skipping 25 matching lines @@
 
   // Whether initialization has been completed.
   bool init_complete_;
 
   ObserverList<PrefStore::Observer, true> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(TestingPrefStore);
 };
 
 #endif  // CHROME_BROWSER_PREFS_TESTING_PREF_STORE_H_