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

chrome/common/persistent_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_COMMON_PERSISTENT_PREF_STORE_H_
 #define CHROME_COMMON_PERSISTENT_PREF_STORE_H_
 #pragma once
 
 #include <string>
 
@@ skipping 47 matching lines @@
 
   // Same as SetValue, but doesn't generate notifications. This is used by
   // PrefService::GetMutableUserPref() in order to put empty entries
   // into the user pref store. Using SetValue is not an option since existing
   // tests rely on the number of notifications generated.
   virtual void SetValueSilently(const std::string& key, base::Value* value) = 0;
 
   // Removes the value for |key|.
   virtual void RemoveValue(const std::string& key) = 0;
 
+  // Marks that the |key| with empty ListValue/DictionaryValue needs to be
+  // persisted.
+  virtual void MarkNeedsEmptyValue(const std::string& key) = 0;
+
   // Whether the store is in a pseudo-read-only mode where changes are not
   // actually persisted to disk.  This happens in some cases when there are
   // read errors during startup.
   virtual bool ReadOnly() const = 0;
 
   // Gets the read error. Only valid if IsInitializationComplete() returns true.
   virtual PrefReadError GetReadError() const = 0;
 
   // Reads the preferences from disk. Notifies observers via
   // "PrefStore::OnInitializationCompleted" when done.
   virtual PrefReadError ReadPrefs() = 0;
 
   // Reads the preferences from disk asynchronously. Notifies observers via
   // "PrefStore::OnInitializationCompleted" when done. Also it fires
   // |error_delegate| if it is not NULL and reading error has occurred.
   // Owns |error_delegate|.
   virtual void ReadPrefsAsync(ReadErrorDelegate* error_delegate) = 0;
 
   // Lands any pending writes to disk.
   virtual void CommitPendingWrite() = 0;
 };
 
 #endif  // CHROME_COMMON_PERSISTENT_PREF_STORE_H_