Remove JsonPrefStore pruning of empty values on write.

base/prefs/json_pref_store.h

Index: base/prefs/json_pref_store.h
diff --git a/base/prefs/json_pref_store.h b/base/prefs/json_pref_store.h
index 9e6c1821834e712496f6cf481fc501a56bbfab30..21fc8f95ac9c0a1863cc665c1ece4133fb14c1d3 100644
--- a/base/prefs/json_pref_store.h
+++ b/base/prefs/json_pref_store.h
@@ -21,8 +21,8 @@
 namespace base {
 class DictionaryValue;
 class FilePath;
-class SequencedWorkerPool;
 class SequencedTaskRunner;
+class SequencedWorkerPool;
 class Value;
 }
 
@@ -58,7 +58,6 @@ class BASE_PREFS_EXPORT JsonPrefStore
   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;