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..8b8b87b6f1c4ad16052582957adc76bafce67e4d 100644
--- a/chrome/common/persistent_pref_store.h
+++ b/chrome/common/persistent_pref_store.h
@@ -65,6 +65,10 @@ class PersistentPrefStore : public PrefStore {
   // 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.