[vm-service] Use consistent success response for _requestHeapSnapshot.

runtime/vm/service.cc

Index: runtime/vm/service.cc
diff --git a/runtime/vm/service.cc b/runtime/vm/service.cc
index 0d134e86371811d3bcae2afc6087014e140ebe10..4c155413379791a8e66d6a217717fdcf49b823c0 100644
--- a/runtime/vm/service.cc
+++ b/runtime/vm/service.cc
@@ -3510,8 +3510,7 @@ static bool RequestHeapSnapshot(Thread* thread, JSONStream* js) {
     Service::SendGraphEvent(thread, roots, collect_garbage);
   }
   // TODO(koda): Provide some id that ties this request to async response(s).
-  JSONObject jsobj(js);
-  jsobj.AddProperty("type", "OK");
+  PrintSuccess(js);
   return true;
 }