Implement handlified String::Flatten.

src/messages.cc

Index: src/messages.cc
diff --git a/src/messages.cc b/src/messages.cc
index 0077d0309f0794b38f7c1ea77f5e11aa46d20f02..f3f51cf44fd15ca60e140bbdbd2142e15f240792 100644
--- a/src/messages.cc
+++ b/src/messages.cc
@@ -180,7 +180,7 @@ Handle<String> MessageHandler::GetMessage(Isolate* isolate,
   // here to improve the efficiency of converting it to a C string and
   // other operations that are likely to take place (see GetLocalizedMessage
   // for example).
-  FlattenString(result_string);
+  result_string = String::Flatten(result_string);
   return result_string;
 }