Convert JsonWriter::Write to taking a const ref for the in-param

sync/internal_api/public/engine/model_safe_worker.cc

Index: sync/internal_api/public/engine/model_safe_worker.cc
diff --git a/sync/internal_api/public/engine/model_safe_worker.cc b/sync/internal_api/public/engine/model_safe_worker.cc
index 9692a78e70f9bdd55f8b33e58b6b874e150f3231..f054e503a88618b97ef2ef18b6157c58782975ec 100644
--- a/sync/internal_api/public/engine/model_safe_worker.cc
+++ b/sync/internal_api/public/engine/model_safe_worker.cc
@@ -24,10 +24,8 @@ scoped_ptr<base::DictionaryValue> ModelSafeRoutingInfoToValue(
 
 std::string ModelSafeRoutingInfoToString(
     const ModelSafeRoutingInfo& routing_info) {
-  scoped_ptr<base::DictionaryValue> dict =
-      ModelSafeRoutingInfoToValue(routing_info);
   std::string json;
-  base::JSONWriter::Write(dict.get(), &json);
+  base::JSONWriter::Write(*ModelSafeRoutingInfoToValue(routing_info), &json);
   return json;
 }