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

chrome/common/persistent_pref_store.h

Index: chrome/common/persistent_pref_store.h
diff --git a/chrome/common/persistent_pref_store.h b/chrome/common/persistent_pref_store.h
index df5a49a7a59457fa2a8378fb3ab1bd075fb66108..57f31ec42e348306c12068d3e6352a9041d178ca 100644
--- a/chrome/common/persistent_pref_store.h
+++ b/chrome/common/persistent_pref_store.h
@@ -65,6 +65,9 @@ class PersistentPrefStore : public PrefStore {
   // Removes the value for |key|.
   virtual void RemoveValue(const std::string& key) = 0;
 
+  // Marks the |key| with empty ListValue/DictionaryValue need to be persisted.

[battre, 2012/04/11 21:55:51]

grammar: the |key| (singular) vs. need (plural)

Marks that the |key| with empty ListValue/DictionaryValue needs to be persisted.

[xiyuan, 2012/04/11 22:36:46]

Done.

+  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.