Remove JsonPrefStore pruning of empty values on write.

components/user_prefs/pref_registry_syncable.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 COMPONENTS_USER_PREFS_PREF_REGISTRY_SYNCABLE_H_
 #define COMPONENTS_USER_PREFS_PREF_REGISTRY_SYNCABLE_H_
 
 #include <set>
 #include <string>
 
+#include "base/callback.h"
 #include "base/prefs/pref_registry.h"
 #include "components/user_prefs/user_prefs_export.h"
 
 namespace base {
 class DictionaryValue;
 class FilePath;
 class ListValue;
 class Value;
 }
 
@@ skipping 101 matching lines @@
 
   // Contains the names of all registered preferences that are syncable.
   PrefToStatus syncable_preferences_;
 
   DISALLOW_COPY_AND_ASSIGN(PrefRegistrySyncable);
 };
 
 }  // namespace user_prefs
 
 #endif  // COMPONENTS_USER_PREFS_PREF_REGISTRY_SYNCABLE_H_